The structures, gas-phase acidities and vibrational spectra of selenoformic O-acid, selenoformic Se-acid and several of their derivatives RCSeOH and RCOSeH (R = H, F, Cl, NH2, CH3) were investigated by DFT calculations. Geometry optimizations and frequency computations were performed at the Becke3LYP level of theory with the 6-31 1+G(d,p) basis set, For all 10 acids studied, the syn conformers are predicted to have the lowest energy. The syn-anti enthalpy difference varies between 0.3 and 9.9 kcal mol(-1), the syn selenol acids being substantially more stable than their syn selenoxo counterparts. The substitution of oxygen by selenium in the selenocarboxylic acids studied leads to increased acidity. The selenol acids are weaker acids than the selenoxo derivatives.