Apparent activation energies for the reduction of NO by CO and H-2 are determined for calcined limestone and CaO surfaces. For calcined limestone, the importance of particle size on the apparent activation energy is shown. For small particle sizes, activation energies of about 20 kcal/mol are observed for both CO and H-2 as the reducing agent. For coarse particles, an apparent activation energy of 32 kcal/mol is found for the reduction of NO by CO. For H-2 as the reducing agent, a low-temperature value of 35 kcal/mol (550-650 degrees C) and a high-temperature value of 16 kcal/mol (650-800 degrees C) are observed. These values are compared to the apparent activation energies for the same reactions determined over three different CaO surfaces. The differences seen are ascribed to the presence of catalytically active second phases in the calcined limestone.