Highly correlated ab initio molecular orbital methodologies have been used in the study of substituent effects on the singlet-triplet gaps of a series of nitrenium ions and their corresponding isoelectronic carbenes. Calculations in solution were carried out with the isodensity polarizable continuum models, IPCM. The results show a net stabilization of the singlet species as a result of electronic density donation of the substituents to a vacant p orbital located in the central atom. It is shown that this "pi-donor-pi-acceptor" dependence of the singlet-triplet gap is more significant in the case of the nitrenium ions, due to the presence of a positive charge. In addition, it was found that the singlet-triplet gaps decrease with the polarity of the solvent, most likely due to stabilizing electrostatic interactions between the solvent and the charge distribution of the singlet.