We have carried out a G3(MP2) investigation of the enthalpies of formation, isomerization, and hydrogenation (Delta(f)H(298), Delta(isom)H(298), and Delta(hyd)H(298)), in the gaseous state, of 30 hydrocarbons related, directly or indirectly, to [3]-radialene (tris(methylidine)cyclopropane). The thermochemistry of [3]-radialene and many of the other molecules presented he re is dominated by molecular strain. We have discussed the use and limitations of several methods of obtaining an estimate of strain energy (enthalpy). Despite its extraordinarily high Delta(f)H(298), [3]-radialene (an isomer of benzene) appears to be slightly stabilized by its radial methylene groups, in opposition to the large destabilization brought about by ring strain. Exocyclic conjugative stabilization may be present in an amount less than 5 kcal mol(-1), but it is difficult to quantify because there is no rigorous way of separating enthalpic effects due to strain from those due to electron conjugation in these calculations.