The pure rotational spectrum of the free radical MgBr has been measured in its (2) Sigma(+) ground electronic state by Fourier transform microwave spectroscopy. Transitions have been observed for both (MgBr)-Mg-24-Br-79 and (MgBr)-Mg-24-Br-81 in the v = 0 and v = 1 vibrational states. Rotational and centrifugal distortion constants have been determined for each isotopomer in each vibrational state. Equilibrium rotational constants have been calculated and an accurate equilibrium bond length has been determined. Spin-rotation constants, for both the unpaired electron and the bromine nuclei, have been calculated along with magnetic and nuclear quadrupole hyperfine constants for the bromine nuclei. From these constants, the electronic structure of MgBr has been investigated and comparisons have been made to similar compounds. The unpaired electron spin density on the bromine nucleus has been found to be very small, suggesting that this is a very ionic compound. However, the Mg-Br bond has been found to have more covalent character than the bond in other alkaline earth monobromides. (C) 1997 American Institute of Physics. [S0021-9606(97)02046-1].