Substituent effects in pyrimidine were studied by X-ray crystallographic methods. The crystal structures of the title compounds were derived from data collected at 107.degree. K. Only high-order data (sin .theta./.lambda. > 0.60) were used for the determination of non-H parameters. The final R-values lie in the range 0.033-0.054 and the ESD in bond lengths are about 0.002 .ANG.. In addition to the influence of substitution on bond angles, small but probably significant changes occur in the ring bonds adjacent to the substitution position. Cl substitution shortened these bonds by 0.009(2) .ANG., whereas introduction of methyl and amino groups causes lengthenings of 0.006(2) and 0.019(3) .ANG., respectively. The electron density difference maps for the 4 compounds were compared.