The replacement of a hydrogen in nitromethane and in phenylnitromethane by the PhCO group has a strong acidifying effect, i.e., PhCOCH(2)NO(2), 5, is 5.8, 6.6, and 8.6 pK(a) units more acidic than CH(3)NO(2) in water, 50% DMSO-50% water (v/v), and 90% DMSO-10% water (v/v), respectively, and PhCOCH(Ph)NO(2), 6, is 2.0, 3.0, and 3.2 pK(a) units more acidic than PhCH(2)NO(2) in the same solvents. A major focus of this paper is an attempt to sort out the relative contributions of resonance, inductive/field, and solvation effects that lead to the increased acidities. To this end rate constants for the reversible deprotonation of 5 by secondary alicyclic amines in water, 50% DMSO-50% water (v/v), and 90% DMSO-10% water (v/v) and for the reversible deprotonation of 6 by secondary alicyclic amines, carboxylate ions, thiolate ions, and aryloxide ions in water, by secondary alicyclic and primary aliphatic amines in 50% DMSO, and by secondary alicyclic anions in 90% DMSO were determined. From Brønsted plots based on these data the intrinsic rate constants (k(o)) for the reactions of 5 and 6 with the various buffer families were obtained and compared with previously determined k(o) values for the deprotonation of CH(3)NO(2) and PhCH(2)NO(2), respectively. An analysis of the changes in k(o) induced by the introduction of the PhCO group, coupled with a comparison of solvent transfer activity coefficients for the transfer of the anions (5(-) and 6(-)) from water to 50% and 90% DMSO with those for CH(2)=NO(2)(-) and PhCH=NO(2)(-), respectively, indicates a substantial increase in the resonance stabilization of 5(-) relative to CH(2)=NO(2)(-) and a corresponding sharp drop in the solvational stabilization by hydrogen bonding from water; the effects on 6(-) are qualitatively similar but quantitatively much smaller. Our study also shows that the intrinsic rate constant for proton transfer from 6 to thiolate ions is higher than for proton transfer to aryl oxide ions and amines. This result indicates that, in contrast to most other proton-transfer reactions, hydrogen-bonding stabilization of the transition state for deprotonation of 6 is not an important factor.