The system Mg x B 2 has been studied to investigate possible nonstoichiometry in MgB 2 . When synthesized at 850 o C, MgB 2 is a line compound with a possible Mg vacancy content of about 1%. Small changes in lattice constants as a function of starting composition result from grain-interaction stresses, whose character is different in the Mg-rich, near-stoichiometric, and Mg-deficient regimes. A small linear decrease of the superconducting transition temperature, T c , in the Mg-rich regime results from accidental impurity doping.