We present a detailed photoluminescence and photoluminescence excitation study of GaAs grown directly on InP substrates by MOCVD. Reliable peak assignments are determined. With these peak assignments, we measure strain and strain uniformity, identify impurities, and assess material quality. Most samples exhibit three distinct spectral features. The two highest energy features are the strain split valence band, the third feature is a carbon impurity. The observed splitting is in good agreement with the value predicted from the thermal expansion mismatch.