We report the doping effect on the intermixing in the GaInAsP/InP multiple quantum well structures with doping densities of 5 x 10(16), 10(17), 5 x 10(17), 10(18), 5 x 10(18)cm(-3) in their wells, respectively. It was found that the lattice mismatch and the energy band gap increased with the annealing temperature and time, and the high doping density in the wells of the multiple quantum well structures degraded the intermixing. The mismatch and the blue shift of the energy band gap in the wells can be explained by the out-diffusion of phosphorus from the InP barriers, and the reduction of the intermixing at high doping can be attributed to the interface roughness and inhomogeneity in the wells caused by high doping. (C) 2004 Elsevier B.V. All rights reserved.