The intermixing process of ultrathin InAs/InP strained quantum well structures by thermal annealing at 730-830-degrees-C is investigated by photoluminescence measurements. Analyzing the results using a microscopic model, the interdiffusion process is characterized by an activation energy close to 3.8+/-2.0 eV, leading to an interdiffusion coefficient close to 7+/-0.5x10(-7) cm2/s at 830-degrees-C.