The effects of hydrogenation on the low‐temperature (5 K) photoluminescence properties of GaAs grown on InP substrate by metalorganic chemical vapor deposition are investigated. An emission band at ∼1.4 eV originating from the GaAs/InP interfacial region shows a 30‐fold increase in intensity relative to the GaAs band‐edge emission after exposure to hydrogen plasma for 30 min at 250 °C. This improvement in intensity is attributed to hydrogen passivation of defects at the heterointerface caused by the large (≊4%) lattice mismatch between GaAs and InP. Annealing the hydrogenated sample at 350 °C nullifies the passivation effect. Further, the 1.4‐eV band shifts to higher energy on annealing the sample in the temperature range 150–450 °C with the hydrogenated sample exhibiting a larger shift than the untreated sample. It is suggested that the annealing‐induced peak shift arises due to modification of the interface and that it is greater in the hydrogenated sample compared to the untreated sample.