We describe an independently valved solid arsenic and phosphorus effusion source for molecular beam epitaxy. A common posteffusion hot zone and orifice ensure constant flux ratios of arsenic, phosphorus, and mixed species in the beam. With this source, we have demonstrated molecular beam epitaxial growth of GaAs1-xPx layers with better than +/-0.4% compositional variation over a 3-in. -diam substrate.