The relative energetics of four XH2O2 (X = C, Si, Ge, Sn, Pb) isomers, dihydroxycarbene, formic acid, dioxirane, and hydroperoxycarbene, were determined using the BLYP and B3LYP density functionals with DZP and TZ2P basis sets, as well as CCSD and CCSD(T) single-point energies at the BLYP/TZ2P optimized geometries. Relative to dihydroxycarbene, formic acid was 41.8 kcal/mol lower in energy while dioxirane and hydroperoxycarbene were 51.3 and 63.6 kcal/mol higher, respectively, with CCSD(T). Furthermore, using an effective core potential (ECP) the dihydroxy congener was shown to be the most stable isomer for X = Si−Pb. The formic acid and dioxirane congeners become increasingly less stable as one descends group 14. Our results show that divalency is preferred for Si−Pb (dihydroxy congeners are the most stable) but the tetravalent formic acid congeners remain more stable than the hydroperoxy congeners, showing that divalency is not universally preferred among these isomers.