We show that optical reflectivity measurements can be used to evaluate the part of a NH, flux which reacts with a Ga-terminated GaN surface or with a Ga-flux simultaneously impinging on the surface, as in standard molecular beam epitaxy situation. At least for temperatures not exceeding 700 degrees C, the ratio of the reacted part of the NH3 flux to the incident flux can be assimilated to the NH:, cracking efficiency. Being nearly zero below a threshold temperature of 450 degrees C, it increases with temperature but remains low (similar to 4%) explaining why an exceptionally high V/III flux ratio is necessary to grow GaN using NH3. (C) 1998 American Institute of Physics.