Room‐temperature electron diffusion lengths (Ln) were determined for lattice matched, p‐type InGaAsP and InGaAs layers grown by liquid phase epitaxy on Sn‐doped (100)‐InP substrates by measuring the variation of the short‐circuit photocurrent as a focused laser beam was scanned along a beveled (ϑ∼1 °) p‐n junction. The hole concentrations, determined by capacitance‐voltage (C‐V) measurements, indicated an almost linear relationship between the hole concentrations in both the quaternary and ternary layers and the amount of Zn in the melt for the growth temperatures and substrate orientation used in this work. The electron diffusion lengths were longest at the lowest hole concentrations and decreased monotonically as the concentration increased. At the lowest doping levels, Ln was 3.5 μm at p = 3×1015 cm−3 for the quaternary and 2.5 μm at p = 1.4×1016 cm−3 for the ternary. At the highest hole concentration (p = 5×1018 cm−3) Ln was 0.13 and 0.83 μm for InGaAsP and InGaAs, respectively.