The nitrosation of methylhydrazine and NN-dimethylhydrazine proceeds through their conjugate acids, and occurs with either the nitrosonium ion or nitrosyl halides as electrophilic reagents. For methylhydrazine the products show that sizeable proportions of both possible conjugate acids must exist in solution. In the presence of a substantial concentration of nitrous acid a secondary reaction can occur, resulting in the destruction of the primary methylnitrosohydrazine, and the formation of methanol, dinitrogen, and nitrogen(I) oxide. A dinitroso-intermediate is probably involved.