Ultraviolet (UV) absorption spectroscopy has been used to monitor the concentrations of gas‐phase reactants participating in the growth of InGaAsP alloys by hydride vapor‐phase epitaxy. Room‐temperature absorption spectra for PH3, AsH3, and HCl as well as high‐temperature (700°C) spectra of InCl, GaCl, PH3,P2, P4, As2, and As4are presented. For the group V species, the UV absorption bands of the hydride, dimer, and tetramer exhibit considerable overlap, but can still be used to determine the approximate concentration of each species. GaCl and InCl exhibit extremely intense, sharp absorption bands at 248 and 267 nm, respectively. This technique has been used to study the effect of PH3pyrolysis on InP growth, and to monitor In/Ga ratio for metal transport from an alloy source. This technique can be implemented on reactor systems without reactor design changes and without disturbing the thermal profile by using ‘‘light pipes’’ to probe concentrations of reactive species during growth.