Abstract: The invention relates to a process for the preparation of N-substituted lactams by reaction of a lactam, which is unsubstituted on the nitrogen, with an organic halide in the presence of at least one solid-liquid phase transfer catalyst, such as a quaternary ammonium salt, and of at least one solid inorganic base, such as an alkali metal hydroxide, and in the absence of solvent. By this process, N-substituted lactams are obtained with good yields and high purity. The absence of solvent makes possible a considerable gain in productivity and an improvement in safety and in regard for the environment.