Equilibrium constants and Gibbs free energy changes for 1:1 charge-transfer complexes between I-2 or ICl and lactams (N-H and N-Me) were determined in solution by means of UV-visible spectroscopic techniques. As expected, ICI-complexes are stronger than I-2 complexes. Ab initio calculations at HF/LANL2DZ* and MP2(full)/LANL2DZ* levels of theory were carried out to investigate the structures of the complexes and the nature of the intermolecular interaction between the lactams and I-2 or ICl. Systematically, the global minimum of the potential energy surface corresponds to the approach of I-2 or ICl to the carbonyl oxygen, with the exception of aziridinone. Two stable conformers, syn and anti with respect to the ring nitrogen atom, were found to be local minima of the potential energy surface. For unsubstituted lactams, the syn conformer is stabilized through a hydrogen-bond interaction with the NH group of the base and becomes the most stable structure. Conversely, for the N-methyl substituted derivatives, where this interaction is not possible, the anti conformer is the most stable one. Experimental free energies in solution and gas-phase theoretical values follow a good linear relationship. Both the experimental results and the ab initio calculations showed that, contrary to lactones, lactams are more basic than cyclic ketones with respect to ICl and I-2.