N-Butylamides of long-chain, unbranched carboxylic acids undergo degradation, isomerization, and (in part) dimerization of the fragments when treated at 0-5° with liquid hydrogen fluoride saturated with boron trifluoride. The compounds studied were the N-n-butylamides of undecanoic, myristic, palmitic, and stearic acid. Two major classes of products were obtained: (1) saturated, branched N-n-butylamides and (2) saturated, branched N,N′-di-n-butyldiamides. The ratio of the former to the latter is 2:1. The products from N-n-butylundecanamide consist mainly of the following N-n-butylamides (relative proportions): 5-methylhexanamide (12.9%), 6-methylheptanamide (25.6%), 5-methylheptanamide (18.5%), 7-methyloctanamide (14.5%), and 6-methyloctanamide (6.5%). The branched diamides are predominantly of chain length Cu12-C15 (acid moiety). Similar results are obtained with N-n-butylmyristamide, N-n-butylpalmitamide, and N-n-butylstearamide when treated under the indicated reaction conditions; however, N-n-butylamides of acid moiety chain length shorter than C10 (e.g., the N-alkyloctanamide) are not affected. The probable mechanism of the reaction is discussed, and it is concluded that the inductive effect of the protonated amide group is important in determining the product distribution.