The complete basis set method CBS-QB3 was used in conjunction with the CPCM solvation model to predict both the absolute and relative pK a's of 12 nucleophilic carbenes in dimethyl sulfoxide (DMSO), acetonitrile (MeCN), and water. Average absolute pK a values in DMSO ranged from 14.4 ± 0.16 for 3-methylthiazol-2-ylidene (12) to 27.9 ± 0.23 in the case of bis(dimethylamino)carbene (11), while values in MeCN were determined to be between 25.7 ± 0.16 (12) and 39.1 ± 0.25 (11). Relative pK a calculations yielded similar results. Calculations in aqueous solution gave pK a's between 21.2 ± 0.2 (12) and 34.0 ± 0.3 (11). Excellent agreement between calculated and experimental pK a's was obtained for the few cases where experimental numbers are available, confirming that this theoretical approach may be used to calculate highly accurate pK a values.