Theoretical heats of formation at 298 K for several alkylsilanes, predicted at the MP2/6-31G(d) level of theory, are compared with recently obtained experimental and additivity values. Excellent agreement is obtained between the ab initio and additivity values and with the more reliable experimental values for acyclic alkylsilanes. The ab initio heats of formation for the silacycloalkanes permit the evaluation of strain energy increments for the additivity scheme. Comparison is made with limited experimental data.