Abstract The pure rotational spectrum of the ground electronic state (X 2Σ +) of the MgOH radical has been observed using millimeter/sub-millimeter direct absorption spectroscopy, as well as that of MgOD, and the less abundant magnesium isotopically-substituted species 25MgOH and 26MgOH. The free radicals were created in a Broida-type oven by the reaction of metal vapor with hydrogen peroxide. The spin—rotation splitting was readily observed in these data; however, the hyperfine structure was only resolved for 25MgOH. The rotational and spin—rotation constants of the radicals have been determined from a non-linear least-squares fit using a 2Σ Hamiltonian. The 25Mg hyperfine constants are also estimated.