Phosphorus incorporation during chemical beam epitaxial (CBE) growth of GaAs1-xPx from triethylgallium, tertiarybutylarsine, and tertiarybutylphosphine is investigated. Reflection high-energy electron diffraction intensity oscillations are used to measure the phosphorus incorporation during the As-limited and the (As+P)-limited growth on a Ga-rich surface. The resulting phosphorus mole fraction is compared with the phosphorus composition measured by x-ray rocking curves on the strained GaAs1-xPx layers grown by conventional CBE with a simultaneous supply of group V and group III elements. The phosphorus incorporation rate during CBE growth is lower than that measured during the group V controlled growth but is still much higher than the incorporation rate reported for molecular beam epitaxy growth using elemental sources.