Examples of the use of transient techniques in NO reduction reactions are presented. The specific reactions studied are NO + NH 3 reaction over crystalline vanadia catalysts and NO + CH 4 reaction over Pd/TiO 2 catalysts. The reactions were investigated by the following step function changes: 14 NH 3 + O 2 → 15 NH 3 + O 2 , 14 NO + NH 3 + O 2 → 15 NO + NH 3 + O 2 , 14 N 16 O → 15 N 18 O, 13 CH 4 + NO → NO, 13 CH 4 + NO → He, 13 CH 4 → NO, CH 4 + 15 NO → CH 4 , and CH 4 + 15 NO → NO + CH 4 . The results have provided information about the differences in the surface residence time of intermediates while also making it possible to differentiate between different reaction pathways in the NO reduction reactions with ammonia and with methane.