A series of halomethyl cations was prepared and studied by C-13 MMR spectroscopy. Their structures and C-13 NMR chemical shifts were also calculated by ab initio/DFT/GIAO-MP2 methods. The delta(13)C of trihalomethyl cations CX(3)(+) (X = Cl, Br, I) and dimethylhalocarbenium ions Me(2)CX(+) (X = F, Cl, Br) correlate well with the Pauling electronegativities of the halogen atoms and delta(11)B of the isostructural and isoelectronic boron halides as well as with the calculated charge of carbon atoms. The CF3+ cation was not observed, but a chemical shift of delta(13)C 150 is indicated by comparison with other experimentally observed trihalomethyl cations. GIAO-MP2 chemical shift calculations gave delta(13)C 169.2 for the CF3+ cation.