We show that optical reflectivity measurements can be used to evaluate the part of aNH3flux 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 °C, the ratio of the reacted part of theNH3flux to the incident flux can be assimilated to theNH3cracking efficiency. Being nearly zero below a threshold temperature of 450 °C, it increases with temperature but remains low(∼4&percent;)explaining why an exceptionally high V/III flux ratio is necessary to grow GaN usingNH3.©1998 American Institute of Physics.