The Nef reaction between isocyanides and acyl chlorides is studied at the M06-2X/6-311+G(d,p) level of theory in toluene. After proving that the reaction follows a concerted mechanism instead of an addition elimination path, we study the influences of the solvent, the isocyanide, the acyl moiety and the leaving group on the energy profile of the reaction. The calculated data can be rationalized with the pK(a) of the leaving group, or more generally with the population of the oxygen lone pairs of the acyl moiety.