Poly(urea-formaldehyde) (PUF) microcapsules loaded with sulfur were prepared by in situ polymerization. Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry and thermogravimetric (TG) analysis were used for the characterization of the obtained microcapsules. The chemical structure of the microcapsules was determined by FTIR. SEM analysis showed that the microcapsules were spherical and that the mean diameter of microcapsules was 42 +/- 1 mu m. Thermal analysis showed that the microcapsules were thermally stable below 247 degrees C. The release properties of sulfur from the PUF microcapsules during the vulcanization of acrylonitrile-butadiene rubber (NBR) were studied by analysis of the vulcanization properties. The results show that at a vulcanizing temperature of 160 degrees C, all of the sulfur entrapped in PUF was released out through the microcracks and pores of the shell, and the curing time of the microcapsule curing system compared with that of the sulfur curing system was longer. The mechanical properties of the NBR vulcanized by sulfur microcapsules were better than those of the NBR vulcanized by sulfur. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 767-773, 2011