Models proposed for risk assessment of chemical mixtures assume no interactions between the chemicals. There are, however, studies indicating that some organophosporous insecticides can inhibit the detoxification of other chemicals in plants thereby enhancing their effect. The present study investigates whether interactions between selected organophosporous insecticides and herbicides can take place in the aquatic algae Pseudokirchneriella subcapitata and the aquatic macrophyte Lemna minor. For both species binary mixtures of the organophosphate insecticides: malathion, endosulfan and chlorpyrifos were tested together with the herbicides metsulfuron-methyl, terbutylazine and bentazone. For mixtures with malathion on algae, dose-response surfaces were made and the results tested against the model of concentration addition (CA) and independent action (IA). The Lemna minor tests showed no indication of synergy for any of the combinations, on the contrary, significant antagonism was found for several of the mixtures. The response surface analysis showed antagonism in relation to both concentration addition and independent action for mixtures between malathion and metsulfuron-methyl and terbuthylazine, while the mixtures with bentazone could be explained with CA. The study shows no indications of synergistic interactions between the tested pesticides, confirming the applicability of CA as a reference model predicting mixture effects of pesticides for aquatic plants and algae.