A recently introduced chemoassay has been used to determine second-order rate constants of the electrophile-nucleophile reaction of 15 α,β-unsaturated aldehydes with glutathione. The respective kGSH values vary for more than 3 orders of magnitude, and are within the range determined previously for 31 α,β-unsaturated ketones and esters. Structure-reactivity analyses yield distinct relationships between kGSH and structural features of the compounds. Moreover, increasing kGSH increases the aldehyde toxicity toward ciliates in terms of 48 h-EC50 values (effective concentration yielding 50% growth inhibition of Tetrahymena pyriformis within 48 h). A respective log-log regression equation including both kGSH and the octanol/water partition coefficient, Kow, yields a squared correlation coefficient of 0.96. Comparative analysis with corresponding data for 15 ketones and 16 esters reveals systematic differences between the three compound classes with regard to the individual contributions of hydrophobicity and electrophilic reactivity to aquatic toxicity. The former is particularly pronounced for aldehydes, while the ester toxicity is largely governed by reactivity, with ketones showing an intermediate pattern that is more similar to the one of esters than of aldehydes. It follows that within the Michael acceptor domain of α,β-unsaturated carbonyls, a distinction between aldehydes and nonaldehydic derivatives appears necessary when employing electrophilic reactivity as a component for the quantitative prediction of their reactive toxicity toward aquatic organisms.