13C nmr spectra have been obtained for a wide range of stable carbonium ions using the indor method. The normal substituent parameters appear to operate for the ions (CH3)213C+X; X = C2H5 is at lowest field (δ13CS2 = – 139.2) followed by X = CH3 and X = H, the incremental 13C shifts being 3.8 and 10.4 ppm, respectively. 13C shifts for the ions X = c-C3H5, X = C6H5, and X = OH show that hydroxyl and phenyl are similar in their abilities to delocalize the positive charge, both being considerably more effective than cyclopropyl. Di- and trihydroxy-carbonium ions show the expected trend to higher field whereas the reverse is found for di- and triphenylcarbonium ions. This is interpreted in terms of the nonplanar character of the triphenylcarbonium ion. Scrambling of the 13C label in the case of the isopropyl cation was found, indicating a protonated cyclopropane intermediate. In an extension of these results to the elucidation of carbonium ion structures, 13C resonance is shown to be a powerful method for differentiating between bridged and equilibrating open-chain carbonium ions. The dimethylisopropyl-, dimethyl-t-butyl-, and methylethylcarbonium ions and the cyclopentyl cation are shown by this method to be rapidly equilibrating degenerate ions while the ethylenebromonium and ethylene-p-anisonium ions are shown to have the bridged structure. The norbornyl cation was found to be bridged, the results being consistent with a corner-protonated nortricyclene structure for the ion. σ delocalization is less is still present in the 2-methylnorbornyl cation. It is essentially absent in the 2-phenylnorbornyl cation. 13C spectra for the tetramethyl-ethylenebromonium ion suggest that the bridged and open-chain ions have comparable stabilities or that partial bridging occurs. © 1969, American Chemical Society. All rights reserved.