Oxygen contamination has been observed in In0.5Ga0.5P and (Al0.23Ga0.77)(0.5)In0.5P films grown by solid source molecular beam epitaxy with elemental phosphorus. Using a conventional Pq cracking zone temperature of 950 degrees C, spike contamination levels as high as 1 x 10(19) cm(-3) were observed at growth interrupted interfaces with the resultant deactivation of silicon doping pulses. By reducing the phosphorus cracking temperature to 700 degrees C, the oxygen level in InGaP was reduced to below the secondary ion mass spectrometry background level of 3 x 10(16) cm(-3). No measurable accumulation of oxygen was observed at growth interrupted interfaces and efficient silicon pulse doping was obtained. InGaP films grown at the lower cracking temperature exhibited improved mobilities and enhanced photoluminescence intensities. An oxygen level in (Al0.23Ga0.77)(0.5)In0.5P Of less than 1.5 x 10(17) cm(-3) was obtained with good mobilities and luminescence. Efficient silicon pulse doping in AlGaInP was demonstrated. The oxygen contamination is in the phosphorus flux and is likely a volatile phosphorus oxide such as P4O6