Polycyclic aromatic hydrocarbons (PAHs) with highly mutagenic and carcinogenic properties are commonly found in the soil environment. Soil contamination by PAHs has become a major health risk issue. PAHs are widespread and occur at high concentrations (hundreds of mg kg^-1) in soils of many countries. Since natural and xenobiotic PAHs present in soil may be absorbed by plants, PAHs can enter human and animal bodies through the food chain/web. Because of the health hazards of PAHs, understanding the distribution of PAH residues in rhizospheric soils is of crucial importance for risk assessment of PAH-contaminated areas. The distribution of PAHs in the rhizosphere affects their fate in the soil-plant system. After diffusion into rhizosphere soil, root exudates gradually disappear as a result of radial dilution and microbial consumption. Because these root exudates are ready carbon and energy sources to bacteria, a bacterial gradient is observed with a greater number of heterotrophs and PAH-degrading bacteria closest to the roots, which may generate a gradient of PAH degradation between the rhizosphere and bulk soil. Recently, it was reported that the residual concentrations of PAHs showed a rising gradient from the rhizoplane to the loosely adhering soil after 40 and 50 d, and were significantly and negatively correlated with the amount of root exudates in the rhizosphere. This was further supported by an in situ observation that concentrations of 11 EPA-priority PAHs in rhizosphere soils increased with the distance (0~9 mm) from the root surface. However, the documented gradient distributions in rhizosphere soils are overwhelmingly about the total concentrations of PAHs as well as other organic compounds. The International Union of Pure and Applied Chemistry (IUPAC) definition reserves the term of bound residues for the parent compound and its metabolites that cannot be extracted from soil using organic solvents. Bound residues have a direct effect on long-term partitioning behavior, bioavailability, and toxicity of the organic contaminants in soil. The formation of bound residue is considered to act as a soil detoxification process by permanently binding compounds into soil matrices, and the bioavailability of bound residues is the final endpoint for risk assessment and regulatory management of organic chemicals in the soil environment. However, there is little information available on the distribution of their bound residues in the rhizosphere. Therefore investigations were made of rhizospheric gradient distribution of bound-PAH residues (reference to parent compounds) in soils on a field scale. In Moleplant ( Euphorbia lathyris L.) and wood sorrel ( Oxalis corniculata L.) fields of yellow brown earth near a petrochemical plant, rhizosphere soils of the plants were sampled including the rhizoplane, strongly adhering soil, and loosely adhering soil, for analysis of content and gradient distribution of PCBR (Parent compound of bound residue) in the rhizosphere. Results show that PCBRs of the ten EPA-priority PAHs were detected in both the rhizosphere and non-rhizosphere soils, about 3.31 mg kg^-1 in concentration in the latter, much higher than in the former (1.07~1.82 mg kg^-1). The concentration of PCBRs increased with the distance (0~9 mm) from the root surface. It is feasible to use rhizosphere effect (R, in percent) to measure the proportion of the decrement of PCBRs in concentration in the rhizosphere as against that in the non-rhizosphere soil, R decreased with increasing distance from the root. Rof the total PCBRs of PAHs in three continuous layers of rhizosphere reached 45.15%~67.66%. R of two-ringed PAHs was the highest (61.18%~93.50%), while R of four- and five-ringed PAHs the lowest (2.39%~6.31%), which indicates that the PCBRs of PAHs with fewer rings are more liable to transformation in the rhizosphere. R of the PCBRs in the rhizosphere of moleplants was found to be relatively higher than that of wood sorrels, suggesting that the rhizosphere of moleplants was more favorable to transformation of PCBRs. The gradient distribution of PCBRs in the rhizosphere is closely related to that of root exudates, while type of PAHs and rhizosphere environment affect significantly distribution of PCBRs. The findings of this work provide some important information on fate of PAHs in the soil environment and are useful in risk assessment of PAHs-contaminated soils and development of strategies for remediation of contaminated areas.