The ammonia synthesis rate over a ruthenium single crystal has been studied in the temperature range 598-898 K. Ammonia was synthesized from a stoichiometric (3H(2) : N-2) gas-mixture at a pressure of approximately 2 bar. The ammonia production rate was measured in two ways : with the reactor isolated (batch mode) and under constant gas flow (flow mode). The overall activation energy was measured to be 101 +/- 4 kJ/mol, and the turnover frequency was 2 x 10(-2) s(-1) at 673 K. After synthesis, the primary adsorbed species was nitrogen and there was evidence of hydrogen in the bulk of the crystal.