Heats of formation of spiropentadiene, spiropentane and bicyclo[1.1.0]but-1(3)-ene have been calculated employing the G2M(RCC,MP2) method. The values obtained using 27, 8 and 10 different working reactions are 150.5, 43.0 and 133.0 kcal/mol for spiropentadiene, spiropentane and bicyclo[1.1.0]but-1(3)-ene, respectively, with the error bars estimated as +/-2 kcal/mol. The results are in close agreement with available experimental data and allow to resolve the persisting uncertainties in the heats of formation of the simplest bicyclic hydrocarbons.