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An improved understanding of pesticide toxicity in natural systems can have important consequences for pesticide management and remediation strategies for contaminated areas. The interaction between humic substances extracted from compost natural organic matter and both organic and inorganic pesticides was evaluated for its effect on the toxicity of pesticides in the aqueous phase. The toxicity of contaminants was measured using the Microtox toxicological bioassay. Solutions containing concentrations ranging from 2 to 42 mg C/l of humic substance extracted from a South Texas compost were added to concentrations of the organic pesticide, chlorpyrifos, and toxicity reduction ranging from 50 to 100% was demonstrated. Different concentrations of arsenic ranging from 0.5 to 5 mg/l were also associated with three different concentrations of humic substances and the arsenic toxicity was consistently reduced by a factor of 100%. These results demonstrate a significant relationship between humic substance interactions with organic and inorganic pesticides, and pesticide toxicity in natural systems, and may also suggest a mechanism for pesticide toxicity reduction in natural waters through compost humic addition for contaminated groundwaters and surface waters.