The chlorination of 1-methylnaphthalene in acetic acid, like that of naphthalene1 and of 2-methylnaphthalene,2 gives products of substitution accompanied by those of addition. The adducts include tetrachloro-derivatives of 1-methyl-, 1-chloro-4-methyl-, and 1-chloromethyl-tetralin; they have been isolated and characterised, and their structures and predominant conformations have been partially elucidated. Reaction paths leading to these products are discussed.