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This work provides a kinetic and experimental investigation of excited radical OH(2Σ+) (also referred to as OH*) in NH 3 H 2 -air flames. A counterflow burner is used to stabilize laminar diffusion flames over wide ranges of ammonia fraction in the fuel blend (0.2 ≤ x NH3 ≤ 0.8) and strain rate (40 ≤ a ≤ 200/s). Using an intensified camera or a spectrometer coupled to a Cassegrain optical system, spatially resolved and spatially integrated OH(A2Σ+-X2Π) chemiluminescence intensities are measured. These data are used to challenge a kinetic mechanism largely developed from existing literature schemes. Measurements and simulations show that two distinct OH* peaks exist in spatially resolved profiles for intermediate ammonia fractions in the fuel blend. Sensitivity analyses identified that reactions N 2 O+ H =N 2 +OH* and H + O + M =OH*+ M , respectively pertinent to NH 3 and H 2 oxidation routes, are responsible for the formation of OH*. The proposed kinetic mechanism gives a remarkable portrait of the empirical data recorded in diffusion flames as well as in premixed NH 3 H 2 -air flames from the literature. Nevertheless, the contribution of reaction N 2 O+ H =N 2 +OH* in the formation of OH* in the peak closest to the fuel side of diffusion flames is consistently overpredicted. Consequently, the frequency factor of the N 2 O+ H N 2 +OH* reaction is adjusted to A = 1.35 × 1014 cm3mol−1s−1, which significantly improves predictions of spatially integrated and spatially resolved OH* intensities for all the diffusion and premixed flames examined. [ABSTRACT FROM AUTHOR]