Boron is potentially useful for strain balancing compressively strained materials such as InGaAs and GaAsBi that are being developed for use in optical and electronic devices. Understanding and improving the incorporation of boron in GaAs is an important first step toward the realization of these strain-balanced systems. Here, we show that the apparent boron incorporation in GaAs, determined from X-ray diffraction measurements, decreases as the substrate temperature is increased, although measurements of the metallurgical concentration of boron remain constant. This implies that boron is incorporating preferentially on non-substitutional sites as growth temperature is increased. The addition of a bismuth surfactant flux not only makes the epilayers smoother, but within a narrow range of substrate temperatures, restores the incorporation of substitutional boron. (c) 2012 Elsevier B.V. Allrights reserved.