High-level ab initio molecular orbital calculations are reported for the global minima on the SiCHn (n = 0-4), SiCHn+ (n = 0-5), SiC2Hn (n = 0-4, 6), and SiC2Hn+ (n = 0-5, 7) potential energy surfaces. The results have been used to calculate standard enthalpies of formation at 298 K for each compound, ionization energies at 0 K, and proton affinities at 298 K for the neutral species. The single-carbon compounds have been investigated at levels of theory up to PMP4SDTQ(fu11)/6-311++G(2df,p)//MP2(full)/6-311G(d,p) and QCISD(T)(full)/6-311++G(2df,p)//MP2(full)/6-311G(d,p); those with two carbons have been investigated at levels up to PMP4(fc)/6-311++G(2df,p)//MP2(full)/6-31G(d,p) and QCISD(T)(full)/6-311++G(2df,p)//MP2(full)/6-31G(d,p). Harmonic frequency calculations, performed on each optimized geometry, established all the structures to be at minima and also provided zero-point vibrational energies. Calculated thermodynamic values at the PMP4 and QCI levels of theory are in good agreement with each other (except where the removal of spin contamination by spin annihilation is inadequate) and are in good agreement with well-established experimental values where such comparisons are possible. Calculated ionization energies are consistently lower than the experimental values for the carbon analogues; proton affinities of the organosilicon species are higher than those of the carbon analogues for which experimental data are available.