The structure and gas-phase acidities of formohydroxamic and silaformohydroxamic acid derivatives R-M(=O)NHOH (R = H, CH3, CF3, and phenyl; M = C and Si) have been studied using the Becke3LYP functional of DFT theory and the two-layered ONIOM (B3LYP/6-311+G(d,p): AM1) method. The calculations showed that the thermodynamic stability of the neutral species and their anions depends on both the type of substituent R and the possibility for competitive existence of O- and N-anions resulting from the monodeprotonation of the hydroxamate and silahydroxamate moieties. The molecules of neutral acids should exist in several forms very close in energy. Thus formohydroxamic, acetohydroxamic, trifluoroacetohydroxamic, benzohydroxamic, trifluorosilaacetohydroxamic, and silabenzohydroxamic acids in the gas phase are N-acids. On the other hand, the N(H)O- anion is more stable in silaacetohydroxamic and silabenzohydroxamic acids, hence these acids in the gas phase are O-acids. The acidity increases in the order: RSi(=O)NHOH < RC(=O)NHOH (R = H, CH3, CF3, and phenyl). Their acidity order is CH3M(=O)NHOH < HM(=O)NHOH < Phe-M(=O)NHOH < CF3M(=O)NHOH (M = C and Si). The highest gas-phase acidity (1336 kJ mol(-1)) has been calculated for trifluoroacetohydroxamic acid. The acidities of phenyl-substituted derivatives computed using the hybrid ONIOM (B3LYP/6-311+G(d,p):AM1) method are in very good agreement with the full DFT ones and this method can be adopted to model large substituted hydroxamic acids.