The use of aqueous-organic two-phase systems in microalgae biocatalyzed processes can improve productivity and overcome lack of water solubility and poor extracellular excretion of many natural products produced by microalgae. In the present work the toxic effect of various solvents belonging to two different homologous series on several marine and freshwater microalgae and one cyanobacteria have been studied, Typical sigmoidal plots were obtained when plotting solvent biocompatibilty versus the Hansh parameter "log P-oct" (logarithm of the solvent partition coefficient in a standard octanol/water system). Tolerance of microalgae was found to be intermediate between that reported for bacteria and for plant cell suspensions. Anabaena, a blue-green alga with more similarities with bacteria than the others algae studied, was found the most tolerant. Good correlation was found between the aqueous critical solvent concentration and the Hansh parameter for all the microalgae studied. The membrane critical solvent concentration for each microalgae was calculated applying Osborne's model. The protective effect of cell immobilization by gel entrapment against solvent toxicity has also been investigated, and surprisingly good results were obtained even for long-term experiments.