The heats of formation and proton affinities of eight simplest oxoborons (RB=O) and sulfidoborons (RB=S, with R = H, F, Cl, and CH3) have been determined using ab initio molecular orbital calculations at the MP4SDTQ/6-311++G(3df,2p) level with MP2/6-31G(d,p) geometries together with zero-point vibrational and thermal corrections. The theoretical values for DELTAH(f,298) (kcal/mol) of the species considered are HBO -62.9; HBOH+ 142.9; FBO -146.1; FBOH+ 61.4; ClBO -96.5; ClBOH+ 103.9; CH3BO -83.5; CH3BOH+ 108.4; HBS 6.4; HBSH+ 220.5; FBS -82.5; FBSH+ 126.3; ClBS -31.6; ClBSH+ 171.6; CH3BS -14.4; and CH3BSH+ 184.1. The proton affinities (PA at 298.15 K, kcal/mol) are HBO 161.3; FBO 159.6; CIBO 166.8; CH3BO 175.3; HBS 153. 1; FBS 158.4; ClBS 164.0; and CH3BS 168.7. The accuracy of the predicted values is expected to be about, or even better than, +/-2 kcal/mol. The reliability of available experimental data has been assessed and the trend in PA's discussed.