Abstract: In order to determine the influence of soil organic matter content on bioavailability ofproducts applied to the soil and their side-effects on soil dwelling beneficial arthropods, a set ofexperiments with chlorpropham and carbosulfan as test products, Bembidion lampros andAleochara bilineata as beneficial insects and pure sand with addition of 3%, 6% and 9% compostas substrate was carried out in the laboratory. Both beetle mortality or reduction in onion flypupae parasitism by rove beetle and pesticide bioavailability were determined and compared.Products were tested at different rates according to standard IOBC methods. Bioavailability ofpesticide residues was determined by chemical analysis by HPLC, comparing total extract of thesubstrate to a CaCl2 aqueous extract that only extract pesticide residues that are not fixed onorganic matter complex.Results showed that toxicity was correlated to tested doses and organic content of the substrate.At the maximum recommended field rate, Chlorpropham lead to 96% mortality of B. lampros and93% parasitism reduction by A. bilineata on pure sand. With addition of organic matter, toxicityrapidly decreased and the effects of the herbicide only reached 3% for B. lampros and 0% for A.bilineata with sand + 9% of compost. Similar results were obtained with carbosulfan at 1% of therecommended field rate, with 50% and 7% mortality for B. lampros on pure sand and on sand +9% compost, respectively. At the same rate, 100% parasitism reduction was obtained for A.bilineata on pure sand, compare to 0% parasitism reduction on sand + 9% compost. Intermediateresults were obtained with sand + 3% or 6% compost. Decrease in toxicity was linked to amountsof organic matter added to the sand and were indicating a strong relationship between effects,applied doses and organic matter content.Pesticide residue analysis confirm that bioavailable doses were negatively correlated with theaddition of organic matter and were strongly related to the applied dose and the organic mattercontent of the substrate. A comparison of dose-response relationship established on pure sand anddose-response relationship established on basis of effects obtained on sand + organic matter andbioavailable doses were indicating that the dose-response were strongly related for the 4 systems (2products x 2 insects). These results confirm that organic matter is a major component of the soil ableto immobilise pesticide residue and reduce their toxicity to beneficial organism. This propriety isdiscussed in regard of testing scheme for soil beneficial (selection of substrate) and in the globalcontext of pesticide use, as the fixation of pesticide on organic matter has probably also a greatimpact on efficacy of products and choice of dose to be applied in the field.