Epoxidation performed with performic acid generated in situ from the couple hydrogen peroxide/formic acid was studied for synthetic polyisoprene latex (LIR-700) and natural rubber (NR) latex. Various parameters of the reaction (nature and concentration of surfactant used, reaction temperature, and quantities of formic acid and hydrogen peroxide used to form formic acid) were studied and the results obtained for the respective latices were compared. At similar epoxidation conditions, the results obtained with LIR-700 and NR latices were shown to be very close. NR and LIR-700 latices showed the same behaviour during the epoxidation performed with perforimc acid. Optimal conditions to carry out epoxidation for the preparation of stable latices were also established. This was achieved when the epoxidation was carried out on latices previously stabilised with 3 p.h.r. of a non-ionic surfactant (Sinnopal NP 307 for example) at a temperature of 60 degree C, with equimolar quantities of formic acid and hydrogen peroxide.