A variety of new 5-alkenyluracils has been prepared in high yields by Wittig olefination of 5-formyl-1-octyluracil, 5-formyl-1,3-dioctyluracil and 5-formyl-2,4-dimethoxy pyrimidine with stabilized and semistabilized phosphorus ylides. The conformation of the products is discussed on the basis of 1 H NMR spectral data.