The treatment of cosmetic wastewaters by Fenton (Fe2+/H2O2) and Fenton-like (Fe3+/H2O2) oxidation has been studied. From batch and continuous experiments it has been proved that both versions of the Fenton process lead to quite similar results in terms of chemical oxygen demand (COD) and total organic carbon reduction although the COD shows a slightly higher rate in the early stages of reaction. COD reductions of around 55% after 2 h reaction time and 75–80% with 4 h residence time were reached in batch and continuous experiments, respectively, conducted at pH around 3, ambient temperature (20 °C), with 200 mg/L of Fe dose and an initial H2O2/COD weight ratio corresponding to the theoretical stoichiometric value. Achieving the locally allowable limit of COD for industrial wastewater discharge into the municipal sewer system takes no more than 30 min reaction time under those conditions by both Fenton systems. However, the Fenton-like process, where iron is fed as Fe3+, would be preferable for industrial applications since the ferric sludge resulting upon final neutralization of the effluent can be recycled to the process. A second-order kinetic equation with respect to COD fitted fairly well the experimental results at different temperatures, thus providing a simple practical tool for design purposes.