2,4-Diamino-5-(3-alkoxy-4,5-methylenedioxybenzyl)pyrimidines, such as, for example, 2,4-diamino-5-(3-methoxy-4,5-methylenedioxybenzyl)pyrimidine, prepared from the corresponding benzaldehydes, are described. The end products are useful as potentiators of the antibacterial activity of sulfonamides, as well as antibiotics.