The use of P2 and As2 beams generated by several different beam sources for the growth of InP, GaAs, and GaxIn1-xPyAs1-y has been investigated. Accommodation coefficients for As2 and P2 were determined for heated GaAs and InP surfaces. It is demonstrated that a source utilizing decomposition of the hydrides over the range 200–2000 Torr and providing a leak of the resulting P2, As2, and H2 molecules into a molecular beam epitaxy (MBE) system can be used for the growth of GaxIn1-xPyAs1-y layers lattice matched to InP. Heterostructure lasers emitting at 1.5 μm with room temperature threshold current densities of 2000 A/cm2 and differential quantum efficiencies of 17%–19% were fabricated to demonstrate the quality of the epitaxy by this method. Initial studies of the cracking of AsH3 and PH3 at low pressures in contact with heated Ta suggest that Ta acts as a catalyst for the decomposition and that low pressure beam sources may also be useful for gas source MBE.