A basicity scale for selenocarbonyl derivatives which covers a wide range of values (60 kcal/mol) has been established through the use of high-level ab initio and DFT calculations. In our theoretical survey we have included selenoformaldehyde and the corresponding BH2, CH3, NH2, F, and Cl mono- and disubstituted derivatives, as well as carbonyl selenide, thiocarbonyl selenide, and selenoketene. With the only exception of selenoketene. which is a carbon base, all selenocarbonyl compounds investigated behave as selenium bases in the gas phase. Selenocarbonyl derivatives are predicted to be equally or slightly more basic than the thiocarbonyl analogues and, therefore, more basic than the corresponding carbonyl compounds. We have also shown, by means of G2-type calculations. that substituent effects on the relative stability of the neutral and the protonated forms of selenocarbonyl series are also rather similar to those estimated, at the same level of theory, for the thiocarbonyl series. For the neutrals these substituent effects are always stabilizing. Protonated species are strongly stabilized by sigma- and; pi-electron donors, while they are destabilized by sigma-withdrawing substituents. This explain the enhanced basicity of the methyl and ar nino derivatives and the. low intrinsic basicity of the halogen derivatives. For the thiocarbonyl series the G2 calculated proton affinities are in very good agreement with the experimental values, which allow us to be confident in our estimates regarding the proton affinities of the selenocarbonyl derivatives investigated. The B3LYP/6-311 + G(3df,2p) estimated proton affinities are slightly higher than the G2 values. The keto-enol isomerization of the methyl, hydroxy, and amino monosubstituted derivatives has been also studied.