Abstract: A method of preparing .omega.-lactams, in particular caprolactam, comprising: a step of premixing cycloaliphatic acids having the formula ##STR1## where n=3-13, with a dehydrating agent; the first step of reaction with a nitrosating agent; the second step of reaction with the addition of a very small amount of water corresponding to a molar ratio U=H.sub.2 O/SO.sub.3 within the 0.1 to 0.9 range; and the step of reclaiming the unreacted cycloaliphatic acid. The method affords improved output from the lactamization reaction and reduced byproducts.