The mechanism of alkylation of 4-quinolone (3) by trialkyl phosphates has been studied by preparative, kinetic, and isotopic methods, as well as by u.v. spectrophotometry. In a kinetically controlled process the transient initial formation of 4-methoxyquinoline (4a) was established; this was subsequently transformed by trimethyl phosphate to a quaternary N-methyl-4-methoxyquinolinium salt (6a). The latter catalyses the transformation of (4a) by intermolecular O→N methyl transfer to the more stable N-methyl-4-quinolone (5a). The end product is a mixture of (5a) and (6a). Their ratio is shifted at higher temperature in favour of (6a). This is due partly to differences in the temperature dependence of the individual reactions and partly to alkylation of (5a) by trimethyl phosphate at higher temperature. A mechanism for the alkylation reaction is proposed.