The O 1s photoabsorption ~ion-yield! spectra of OH (2Pi ,v950) and OD (2Pi ,v950) radicals have been studied in the gas phase for the first time. Spectroscopic parameters for the O 1s!p excited 2S1 state of OH and OD, observed at ;525.8 eV, have been obtained from a Franck– Condon analysis comparing the spectra of both molecules. The lifetime width of this state has been measured as G5147611 meV and G515667 meV for OH and OD, respectively. Excellent quantitative agreement is found between the experimental spectroscopic parameters re , ve , and T0 , measured in this work for the 2S1 state and recent theoretical results obtained for OH by high-level ab initio calculations. The spectroscopic parameters predicted by the equivalent-core model are close to the experimental values within a few percent. This model therefore provides an accurate description of the lowest-lying excitation process in both OH and OD molecules.