Results are presented on the first state‐resolved optical detection of As2in a molecular‐beam epitaxy (MBE) reactor. Using the technique of laser induced fluorescence (LIF) on theA 1∑+u–X 1∑+gtransition, the gas phase populations of vibrational and rotational states of As2emanating from a commercial As4oven‐cracker source are probed. A Boltzmann fit of the populations of the first four vibrations indicates that these states are thermalized to the source temperature (source: 1050±25 K; vibrations: 1020±100 K). Likewise, the rotational manifold of each vibration is consistent with thermalization at the same temperature. The sensitivity of the LIF technique is sufficient to characterize a flux of 3×1014As2cm−2 s−1. Therefore, the method is capable of being aninsitureal‐time MBE diagnostic. Optical detection may provide complementary information to reflection high‐energy electron diffraction (RHEED) that would not otherwise be available. Future applications of this technique may lead to new insights into epitaxial growth processes.