Quantitative thermodynamic stability scales of organolithium compounds can be derived from measurements of tin-lithium exchange equilibria. A DeltaG(eq) scale of alpha-oxy- and alpha-aminoorganolithium compounds was established, and quantitative stabilization effects of O-alkyl, O-alkoxyalkyl, O-carbamoyl, N-carbamoyl, and O-carbonyl groups of the alpha-carbanion are presented. It has been found that an alpha-oxycarbanion is far better stabilized by a carbonyl group as the O-substituent than by an alkyl or alkoxyalkyl group, while the anion-stabilizing effects of the different O-carbonyl substituents are comparable. An N-carbamoyl group was found to have a somewhat higher stabilizing effect than its O-carbamoyl counterpart. NMR data are presented that show that benzylic N-or O-substituted carbanions have highly planarized structures where the negative charge is highly delocalized. The stability data obtained from the tin-lithium exchanges can be easily converted into "effective pK" data that are useful for predicting the acid-base behavior of this type of organolithium species.