Auger depth profiles of InGaAsP‐InP heterojunctions grown by liquid phase epitaxial techniques under different lattice‐mismatch conditions have been measured. Results are presented for samples with Δa/a less than ±0.03, and equal to +0.20 and −0.11% grown from solutions with XlGa=0.40%, XlP=0.36%, and XlAs=3.53, 3.78, and 3.40%, respectively. Only small differences in the chemical transition width of InP‐InGaAsP and InGaAsP‐InP interfaces (the last grown layer is listed first) have been observed when the lattice mismatch is less than ±0.03%. However, the width of the chemical transition region of the InP‐InGaAsP interface increases more rapidly than that of the InGaAsP‐InP interface as the lattice mismatch increases. This result can be understood in terms of the growth kinetics at the heterojunction interfaces.