AbstractPhotolysis of 2‐dimethylamino‐5, 6‐dimethylpyrimidin‐4‐ol (I) in degassed aqueous solution yields two isomeric photo‐dimers which are formed by dimerisation across the 5, 6‐double bond of I. The yield of dimers is dependent upon the initial concentration of I and on the temperature; this can be explained in terms of a mechanism whereby pyrimidinol aggregates react to give dimers. The formation of aggregates can account for the fact that the quantum yield for the overall reaction of I was higher in degassed acetonitrile than in degassed aqueous solution, it can also account for four photo‐dimers being formed in the former solvent and only two in the latter solvent. The photo‐dimers revert to I when heated or on photolysis. That photodimers are not formed under natural conditions in the environment is considered to be partly because oxygen may react competitively with photo‐excited pyrimidinol species and so p
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