The photochemical behaviour of Trimethoprim, a trimethoxybenzylpyrimidine used as bactericide, in solution has been investigated. Direct irradiation causes a slow reaction (e.g. in methanol 3 10 ÿ4). The molecule is quite sensitive to benzylic hydrogen abstraction by n* ketones yielding dimers in the absence of oxygen or oxidised products (the corresponding benzoylpyrimidine and trimethoxybenzaldehyde) in its presence. The drug is also sensitive to singlet oxygen, likewise undergoing oxidative cleavage, though ineffciently. The reported photolability of drug preparations is probably due to sensitised oxidation by impurities or other components.