We present the results of a defect study of liquid‐encapsulated Czochralski grown InP substrates, and InGaAsP grown by liquid‐phase epitaxy using the technique of transmission cathodoluminescence (TCL). The TCL image of the dislocations permits an accurate measurement of the dislocation density and an estimate of the minority‐carrier diffusion length. In addition to the dislocation density, TCL examines material quality by imaging growth‐related defects, e.g., growth striations in InP and stacking faults in InGaAsP.