N-oxides of benzylpyrimidines of the formula ##STR1## wherein R.sup.1, R.sup.2 and R.sup.3 are as set forth hereinafter, are described. The 2,4-diamino-5-(4-bromo-3,5-dimethoxybenzyl)-pyrimidine is also described. The compounds referred to above are useful as potentiators of the antibacterial activity of sulfonamides.