We have used in situ reflection high-energy electron diffraction (RHEED) to monitor the composition of SrTiO3 (STO) thin films during growth by reactive coevaporation. This is accomplished through careful calibration of the relation between the Ti content and the STO lattice constant determined by RHEED. X-ray diffraction analysis of the films indicates that the rockin-curve and 20-scan peak widths of the STO (002) reflection are significantly improved after adopting this RHEED monitoring technique. This method thus results in improved epitaxial growth of STO thin films with reduced compositional fluctuations. (C) 2004 American Vacuum Society.