Gaseous, liquid and matrix-isolated trimethyldioxorhenium(vii), 1, has been characterised (i) by the vibrational spectra of the three isotopomers, (CH3)3ReO2, 1-h9, (CD3)3ReO2, 1-d9, and (CHD2)3ReO2, 1-d6, recorded between 4000 and 400 cm^{â 1}, and (ii) by density functional theory (DFT) calculations. The spectra have been analysed with particular reference to the ν(CH) modes, whence it appears that the three methyl groups all have the same asymmetric geometry with one weak and two strong Câ H bonds irrespective of the sites they occupy in the distorted C3ReO2 trigonal bipyramid. IR spectroscopic measurements have been used to chart the reactions activated by irradiating 1-h9, 1-d9, or 1-d6 isolated in a solid argon matrix with broad-band UV-visible light (200 â ¤ λ â ¤ 800 nm). The primary change is thus shown to involve the elimination of methane to afford the novel methylidene-rhenium(vii) compound H2C.dbd;Re(CH3)O2, 2, but a secondary change becomes evident on continued photolysis, probably resulting in the formation of the bis(methylidene) derivative (H2C)2Re(O)OH, 3. The results are discussed in relation to the photochemistries of other alkyloxorhenium compounds.