This study sought to synthesize an in situ epoxidized natural rubber (NR) from 20% dry rubber content latex stabilized by nonionic surfactant, 5 phr of Terric 16A16, in the presence of hydrogen peroxide and formic acid at the temperature of 50'C. The molar ratios of H,C, and HCOOH to isoprene unit were equal, 0.75: 0.75. Reaction was carried out for 3 to 8 h. This reaction yielded products of various epoxide contents depending on reaction time. Based on DSC characterization, epoxide contents of the in situ epoxidized natural rubbers (ENRs) were about 22-39 mol %. Tensile properties and tear resistance of the in situ ENRs were equal to, or better than, those of NR and commercial ENRs. The in situ epoxidation improved resistance to petroleum ether, but not to toluene. Changes in volume and weight of specimens immersed in ASTM no. 3 oil and automobile oils (various trade names: Shell engine oil, Shell gear oil, and Toyota motor oil) exhibited significant decrease after epoxidation, except in Shell brake fluid. Similar results were obtained from tensile testing of the oil-immersed specimens. Tensile strength and elongation at break of the in situ ENRs were much higher than those of NR after immersion in those oils at room temperature for 7 days, except the immersion in brake fluid. Improved oil resistance of the in situ ENRs under severe condition was obtained in gear oil. (C) 2003 Wiley Periodicals, Inc.