Dielectric relaxation measurements were performed on two series of aryl-aliphatic copolyamides. The repeat unit of the first series was made of 1 or 1.8 lactam-12 sequences, a tere- or iso-phthalic moiety, and a 3,3＇-dimethyldicyclohexylmethane unit in regular order. The second series was obtained from condensation of terephthalic acid onto 1,5-diamino-2-methylpentane. The dielectric relaxation measurements demonstrated the existence of several secondary relaxations. The comparison of the results obtained from the two series of samples permitted the assignment of gamma, beta and omega relaxations, observed in the first series of compounds, to motions of non-conjugated C=O groups, C=O groups located between a phenyl ring and a flexible lactam-12 or methylpentane sequence, and C=O groups situated between a phenyl ring and a rigid cycloaliphatic moiety, respectively.