The first arsenide/phosphide heterojunctions grown by conventional molecular-beam epitaxy with solid-source valved crackers are reported. The use of needle valves to switch the group V fluxes allows Ga0.52In0.48P/GaAs heterojunctions to be grown with virtually no intermixing between layers. Photoluminescence from quantum wells as narrow as 40 Å has also been observed. For comparison, conventional mechanical shutters were used to switch the group V fluxes, and the resulting layers were severely intermixed. The relative incorporation coefficients of the arsenic and phosphorous group V species were found to be temperature dependent, but there was no difference as a result of switching from tetrameric to dimeric arsenic.