Large basis set coupled cluster calculations, with corrections for core/valence, atomic spin-orbit, and scalar relativistic effects, have been used to determine the atomization energies of furan ((1)A(1), C4H4O), tetrahydrofuran ((1)A, C4H8O), and the THF-2-yl ((2)A, C4H8O) and THF-3-yl ((2)A, C4H8O) radicals. For furan and tetrahydrofuran, where gas-phase experimental data is available, the level of agreement between experiment and theory is very good. The 0 K heats of formation (kcal/mol) for the four systems are Delta H-f(furan) = -4.6 +/- 0.5 (calcd) vs -5.2 +/- 0.2 (exptl), Delta H-f(tetrahydrofuran) = -37.6 +/- 0.7 (calcd) vs -37.6 +/- 0.2 (exptl), Delta H-f(THF-2-yl) = 5.1 +/- 1.0 (calcd), and Delta H-f(THF-3-yl) = 8.9 +/-1.0 (calcd). At 298 K the comparable values are Delta H-f(furan) = -7.7 +/- 0.5 (calcd) vs -8.3 +/- 0.2 (exptl), Delta H-f(tetrahydrofuran) = -44.0 +/- 0.5 (calcd) vs -44.0 +/- 0.2 (exptl), Delta H-f(THF-2-yl)= -0.5 +/- 1.0 (calcd) and Delta H-f(THF-3-yl) = 3.6 +/- 1.0 (calcd). The principal limitation on the accuracy of the composite coupled cluster approach followed in this work is the high cost of large basis set calculations on chemical systems that lack exploitable elements of symmetry. Three parametrized methods, G2, G3, and CBS-Q, were also found to be in good agreement with experiment.