The variation in the intensity of the excitonlike luminescence lying 5–10 meV below the free‐exciton energy value specific to molecular beam epitaxially grown GaAs was studied under various growth conditions. The intensity of the luminescence increases with increasing CO in the background and/or decreasing As4/Ga flux ratio. Fourier transform infrared spectra indicated the existence of polynuclear carbonyls in the GaAs. The excitonlike luminescence may be caused by radiative recombination of excitons bound to CO incorporated from the ambient on an As site.