The impact ionization characteristics of (Al x Ga 1- x ) 0.52 In 0.48 P have been studied comprehensively across the full composition range. Electron and hole impact ionization coefficients (<inline-formula> <tex-math notation="LaTeX">\alpha </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">\beta </tex-math></inline-formula>, respectively) have been extracted from avalanche multiplication and excess noise data for seven different compositions and compared to those of Al x Ga 1- x As. While both <inline-formula> <tex-math notation="LaTeX">\alpha </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">\beta </tex-math></inline-formula> initially decrease gradually with increasing bandgap, a sharp decrease in <inline-formula> <tex-math notation="LaTeX">\beta </tex-math></inline-formula> occurs in (Al x Ga 1- x ) 0.52 In 0.48 P when <inline-formula> <tex-math notation="LaTeX">{x} > 0.61 </tex-math></inline-formula>, while <inline-formula> <tex-math notation="LaTeX">\alpha </tex-math></inline-formula> decreases only slightly. <inline-formula> <tex-math notation="LaTeX">\alpha </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">\beta </tex-math></inline-formula> decrease minimally with further increases in <inline-formula> <tex-math notation="LaTeX">{x} </tex-math></inline-formula> and the breakdown voltage saturates. This behavior is broadly similar to that seen in Al x Ga 1- x As, suggesting that it may be related to the details of the conduction band structure as it becomes increasingly indirect in both alloy systems.