(69/71)Ga atoms were reacted with 1,2-epoxybutane and its isotopomers, 1,2-epoicybutane-1,1-d(2) (CH(3)CH(2)(CHOCD(2)) over bar) and 1,2-epoxybutane-2-d(1) (CH(3)CH(2)(CDOCH(2)) over bar), under matrix-isolation conditions. The novel gallaoxetanes CH(3)CH(2)(CHCH(2)GaO) over bar and CH(3)CH(2)(CHCH(2)OGa) over bar, resulting from the insertion of the metal atom in the C(1)-O and C(2)-O bonds, respectively, of the 1,2-epoxybutane, were detected by EPR spectroscopy. The Ga and H hyperfine interaction (hfi) values of the gallaoxetanes, calculated using a DFT method, were used to help assign the EPR spectra. A third Ga-centered species, detected at 190 K, underwent spectral changes similar to those of the C(2)-O insertion product upon isotopic substitution of the 1,2-epoxybutane. Although the Ga hfi for this species was 36% smaller than that of the C(2)-O insertion product, the values for the H hfi were similar, suggesting that the carrier of the spectrum was the C(2)-O insertion product where Ga was perturbed by the matrix constraints. The alkyl radical CH(3)CH(2)(center dot CH)CH(2)OGa, resulting from ring -opening at the C(2)-O bond of 1,2-epoicybutane, was observed at temperatures below 150 K. This radical has been implicated in the formation of the C(2) 0 insertion product The unusually small value found for two of the beta-hydrogens of the alkyl radical is discussed.