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The self-association and tautomerism of (E)-isatin-3-4-phenyl(semicarbazone) and (E)-N-methylisatin-3-4-phenyl(semicarbazone) were investigated in solvents of various polarity. In weakly interacting non-polar solvents, such as CHCl3 and benzene, phenylsemicarbazone concentrations above 1×10−5 mol dm−3 result in the formation of dimers or higher aggregates of E-isomers and . This aggregate formation prevents room temperature E–Z isomerization of and to more stable Z-isomers. In contrast to the situation in non-polar solvents, E–Z isomerization from the monomeric form of phenylsemicarbazone and E-isomers occurs in highly interactive polar solvents including MeOH and DMF only at temperatures above 70 °C. Moreover, decrease in phenylsemicarbazone concentration below 1×10−4 mol dm−3 in these highly solute–solvent interacting systems leads to aggregate dissociation, and a new hydrazonol tautomeric form with a high degree of conjugation predominates in these solutions. Theoretical calculations confirm obtained experimental results. Copyright © 2013 John Wiley & Sons, Ltd.