Rotational analyses of the first four bands in the ν 1 progression of the C ̃ 1B 2← X ̃ 1A 1 transition of SeO 2 at 312.7 nm are reported. The gas phase sample, which contained selenium isotopes in natural abundance, was formed in a free jet expansion from a heated nozzle source. The rotational constants for the C ̃ 1B 2 state show a rather erratic dependence on v 1 ′, consistent with the identification of small, local perturbations in the rotational structure. The A-rotational constant shows a particularly irregular dependence on v 1 ′. The r 0 structure was therefore determined from the B and C values of 80SeO 2 to be: r 0=170.909(72) pm and θ 0=103.954(7)°. The increase in r 0 and decrease in θ 0 on electronic excitation are consistent with the molecular orbital descriptions of the C ̃ 1B 2 and X ̃ 1A 1 states of SeO 2. A strong hot band underlies the 1 0 3 band and somewhat devalues a earlier rotational analysis of this band by King and McLean [J. Mol. Spectrosc. 52 (1974) 154]. The isotopic shift of the band origins is also somewhat irregular. A comparison with the analogous C ̃ 1B 2← X ̃ 1A 1 transition of SO 2 is presented.