Recycling aquatic microecosystems consisting of three subsystems with a total volume of 7.5 L were used to study the effects of the insecticide chlorpyrifos. The poorly soluble chlorpyrifos was dosed dissolved in 0.5 ml of acetone. Acetone was found to be responsible for some of the observed effects. It caused a decrease of the pH by approximately two units. The decrease started 2 weeks after the introduction, and recovery to the pretreatment level took several months.