High-quality In0.5Ga0.5P and In0.5Ga0.5P0.99As0.01 epitaxial layers on the GaAs (100) substrate have been grown by liquid phase epitaxy (LPE). Special attention is paid to the deep level transient spectroscopy (DLTS) and transient photoresponse in these samples. It is found that the epitaxial layers grown under the optimum P vapor pressure are free from deep levels and show a much longer photocarrier lifetime. We suggest that the deep level may be attributed to interstitial P atoms. From the photocarrier lifetime of different samples, we can conclude that the interstitial P atoms act as recombination centers.