Laser induced fluorescence spectra of the [17.6]2.5–X2.5 0–0 band of iridium monoxide, IrO, have been obtained at high resolution (<40 MHz). The hyperfine structure due to 193Ir (I = 3/2) and 191Ir (I = 3/2) is well resolved enabling determination of magnetic (h5/2) and nuclear electric quadrupole (eQq0) hyperfine parameters for the [17.6]2.5 (v = 0) and X2.5 (v = 0) states. The dominant configuration of the ground state is determined by comparing the observed molecular hyperfine parameters with values calculated for several possible ground state configurations using atomic hyperfine parameters. A perturbation calculation suggests that the X2.5 state results from the mixing of the Ω = 5/2 component of the predicted ground 4Δi state with the Ω = 5/2 components of two low-lying excited 2Δi states.