Kinetic studies of the para-nitrosation of phenol, anisole, and their [p-2H1]-analogues by sodium nitrite in aqueous HClO4 are reported. With anisole, ether cleavage of the initial product occurs rapidly to give p-nitrosophenol. The rate for phenol is virtually constant below 1M-HClO4 but thereafter increases rapidly with acidity to reach a maximum in 7·5M-HClO4. A similar rate–acidity profile is observed with anisole, apart from the absence of an acid-independent pathway at low acidities. Both phenol and anisole reactions show substantial primary deuterium isotope effects. The data suggest that aromatic nitrosation proceeds via an SE2 mechanism in which slow expulsion of the aromatic proton is a characteristic feature, and that dienone intermediates lie on the reaction co-ordinate for phenol. This limits deduction as to the nature of the nitrosating agent, but there is good evidence for reaction via NO+ above 5M-HClO4.