The E. S. R. spectra observed during the decomposition of acylarylnitrosamines in four series of solvents have been investigated, and the mechanisms of the reactions discussed in terms of the radicals involved. For aromatic hydrocarbons two signals were detected. The first was that due to the (N-phenylacetamido)phenyl-nitroxide pi-radical, as reported by Chalfont and Perkins), while the second was attributed to the phenyldiazotate o-radical. The structure of these radicals is discussed in the light of a study of the spectra resulting from variation of the acyl and aryl groups of the nitrosamide and of theoretical calculations using the McLachlan and CNDO methods. The mechanism originally proposed by Ruchardt and Freudenberg is preferred to that of Perkins, in view of the failure to detect the chain-carrier radical of the latter scheme in some of the solvents, the intensity of the phenyldiazotate signal remaining constant throughout. For ethers the two signals observed were assigned to (N-phenylacetamido) phenylnitroxide and (N-phenylacetamido)-1-alkoxyalkylnitroxide resulting from the addition of phenyl and 1-alkoxyalkyl radicals to the nitroso group of the nitrosamide. The mechanism proposed, which is also applicable to the reaction in alcohols, is based on that currently acceptable for the decomposition of diazonium salts in ethers. Whenever possible assignments have been made for the various other signals detected and the mode of formation of the radicals discussed.