Nonionic surfactants are largely employed in the pharmaceutical, cosmetic, and textile industry. The sensitivity of polyoxyethylene chains to autoxidation is studied because it is responsible for unpleasant effects such as discoloration, degradation of products, and unpleasant odors. In the present work, thermal stability of polyoxyethylene and polyoxyethylene/propylene nonionic surfactants has been studied by employing together differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). First of all, the thermal stability of three monodispersed surfactants (laurilic alcohol plus 4, 6, or 8 EO units) has been studied. Then, the behavior of these products has been compared with those of commercial surfactants containing the same alkyl chain coupled with idrophylic chains of different lengths, containing EO units alone or both EO and PO. The effect of the alkyl chains has also been examined in the same way. In all cases, autoxidation starts at about 100°C with a net heat production proportional to the number of adducts and equal to about (1.5–1.7) × 105 J/adduct. Heat evolution stopped at about the same temperature, independently of the hydrophylic chain length. A residual of 10–30% by weight decomposes, at higher temperatures, with a very small heat evolution. The interpretation of these and other phenomena observed conclude the paper.