Zn diffusions from spin‐on films have been carried out inton‐InP/p+‐InGaAs/n‐InP heterostructures, which were grown by metalorganic vapor phase epitaxy for heterojunction bipolar transistors with Mg as apdopant. After diffusion, Mg was completely substituted by Zn and enriched in the spin‐on film. In the presence of Mg, the indiffusion of Zn is strongly enhanced. By varying doping levels and diffusion conditions, the underlying mechanism is studied and compared to recent experiments with Be‐doped AlGaAs/GaAs heterostructures.