Dynamic EPR spectroscopy has been used to investigate the inversion of 1,3-dioxolan-2-yl and [2-D]-1,3-dioxolan-2-yl radicals. The former shows no detectable line broadening down to 93 K, which implies "instant" inversion on the EPR time scale, but the latter shows a temperature-dependent EPR spectrum from which the inversion rate constants have been deduced. These rate constants give rise to a curved Arrhenius plot which, together with the large primary isotope effect, indicates that the inversion proceeds by quantum-mechanical tunneling. To investigate the transition quantitatively, an ab initio UHF-MO calculation has been carried out, yielding the molecular geometry and the normal-mode frequencies in the equilibrium and transition state, as well as an inversion barrier height of ca. 2500 cm -1. The inversion is found to be dominated by two vibrational degrees of freedom, namely, a ring-bending mode in addition to the CH α-bending mode. The potential of the latter mode is approximated by a modified quartic double-minimum potential. The former mode is treated semiclassically: it modifies the quartic potential via an anharmonic cross-term yielding a series of quartic potentials, one for each state of the ring-bending mode, weighted by the Boltzmann population of the state. The observed rate constants can be fitted accurately to the CH α-level splittings generated by this model. The resulting effective one-dimensional inversion barrier compares favorably with the corresponding barrier deduced quantum-chemically.