A polarity-sensitive probe was used to assess the thermal stability of aggregates of six nonionic surfactants in aqueous solution. The I-1/I-3 peak ratio of the pyrene emission spectrum, which varies with solvent polarity, was taken as an indicator of the probe environment and hence of the surfactant aggregates in which this probe was sequestered. Aggregate stabilities of surfactants in the Triton, Igepal, and Brij series in premicellar, micellar, and supramicellar (clouded) solutions were considered. Temperature-induced clouding was not found to have a measurable effect on the environment of the sequestered probe. Fully developed micelles exhibited a near-linear decrease in pyrene I-1/I-3 with temperature, similar to the changes observed in pure benzene solvent. In the premicellar and early micellar stages, however, thermal stabilities of aggregates varied significantly with the structure and size of both the hydrophobic and hydrophilic moieties of the surfactant molecules. The least stable assemblies were formed by surfactants with short polyoxyethylene chains and highly branched aliphatic segments.