The acidic nitrosation chemistry of nine acyclic secondary and tertiary amidines (Ph-N=C(R(1))NR(2)R(3); R(1) = H, CH(3), Ph; R(2), R(3) = H, Ph or (CH(3))(2) or C(CH(2))(4)) and several N-acylamidines was investigated. The principal nitrosation products were amides derived from the amino moiety and compounds derived from the benzenediazonium ion, which was independently trapped for quantitation in several cases. Tertiary amidines also produce nitrosamines in minor, but significant, yields. The benzamidines did not react, and the N-acylamidines hydrolyzed much more rapidly than they nitrosated. The data support the hypothesis that the reaction occurs by nitrosation on the imino nitrogen, followed by the addition of H(2)O to give a tetrahedral intermediate (alpha-hydroxynitrosamine) for which the main decomposition pathway generates an amide and a diazonium ion. In the case of the pyrrolidine-derived amidines, about 25% of the decomposition results in cleavage of the amine moiety, which nitrosates to give N-nitrosopyrrolidine. Pseudo-first-order rate constants for amidine nitrosation in aqueous acetic acid with excess nitrite at 25 degrees C ranged from (3 to 106) x 10(-5) s(-1), while the amidine basicity ranged over 5 pK(a) units. Rate constants corrected for amidine basicity showed the pyrrolidine derived amidines to be most reactive. The lack of benzamidine nitrosative reactivity is attributed to a very slow rate of H(2)O additon to the N-nitrosoamidinium ion and reversible nitrosation.