Effect concentrations for aquatic baseline toxicity generally decrease with increasing log octanol-water partition coefficient (Kow) values of up to 5 to 6, whereas less is known about the baseline toxicity of organic chemicals with log Kow values above 6. A physicochemical analysis of the dissolution process for organic chemicals was combined with reported baseline toxicity data, leading to the following conclusions. First, no absolute hydrophobicity cutoff exists for baseline toxicity at a log Kow value of 6, because aquatic baseline toxicity for fish and algae was observed for chemicals with log Kow values greater than 6.5 and with effect concentrations less than 10 μg/L. Second, the baseline toxicity of hydrophobic organic substances was exerted at a relatively constant chemical activity of 0.01 to 0.1. Finally, organic chemicals with high melting points cannot provide sufficient chemical activity to exert baseline toxicity when considered as individual, pure chemicals. However, such substances are still expected to contribute to baseline toxicity when part of a complex mixture.