The aggregative stability of natural rubber ( N R ) latex stabilized by nonionic, fatty alcohol ethylene oxide condensate surfactant (Verol S-15) was studied in the presence of formic acid. Natural rubber latex having a n additional 3-5 phr surfactant is stable a t pH 2 when acidified by formic acid and can be successfully subjected to the subsequent in situ epoxidation. Using a large ratio of HzOz to isoprene unit (0.4-4.0 M / M ) and large excess of HzOz to HCOOH (3-13 M / M ) in reaction, the in situ epoxidation can be performed with reaction mixture having dry rubber content up to 35-37% at room temperature or up to 28% at 50ยฐC. This reaction leads to products of large range of epoxide contents and characterized by absence of side-ring opening groups, proved by IR and 'H-NMR analysis. By infrared spectroscopy it was demonstrated a good correlation between the absorbances of the residual double bonds and of the epoxide groups versus the epoxide content in the range from 0 to approx. 60 mol % level. The solubility and gel content were studied in various solvents, demonstrating that epoxidation leads to increasing gel content at high epoxide levels and to enhancing sensitivity of the product toward polar solvents. By GPC, it was shown for the soluble fraction that epoxidation was accompanied by reducing the molecular weight and by change of the MWD compared with the original NR.