The sorption enhancement of o-nitroaniline on a soil by the addition of a mixture of a cationic surfactant (Cetyl Pyridinium Chloride, CPC) and a nonionic surfactant (Polyethylene Glycol Mono-4-nonylphenyl Ether. P0707), as compared with that by individual CPC and P0707, was investigated to examine their potential application in soil retarding for organic contaminants. The maximum sorption coefficients (K-d*) of o-nitroaniline on modified soils by individual CPC and P0707 were 110 mL g(-1) at an added CPC dose of 6000 mg L-1 and 12.5 mL g(-1) at an added P0707 dose of 7000 mg L-1, respectively. They were approximately 78 and 8.87 times higher than the sorption coefficients (K-d) of o-nitroaniline on the unmodified soil (1.41 mL g(-1)), suggesting that CPC and P0707 can enhance the sorptive capacity of soil for o-nitroaniline. The maximum K-d* value of o-nitroaniline on modified soils by the mixture of CPC and P0707 was 144 mg L-1 (at CPC dose of 10,500 mg L-1 and P0707 dose of 6000 mg L-1), which is higher than that on the CPC (or P0707) modified soils and approximately 102 times higher than that on the un-modified soil. Therefore, mixed cationic-nonionic surfactants could be better than individual ones in soil modification to enhance retention of organic contaminants. (C) 2010 Elsevier B.V. All rights reserved.