The epoxidation of low-molecular-weight natural rubber isolated from Euphorbia lactiflua latex was accomplished. The reaction was performed using formic peracid formed "in situ" from hydrogen peroxide and formic acid. Different temperatures, ratio of H2O2 to isoprene units and H2O2 to HCOOH were used. The products were characterized by infrared spectroscopy (IR), 1(H) and C-13 nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and elementary analysis. H-1-NMR was used quantitatively to calculate epoxidation levels. Maximum epoxidation reached was 89.4%. DSC results record a linear relationship between percent epoxidation and Tg.