In this work we investigate metalorganic chemical vapor deposition (MOCVD) diffusion of zinc into undoped InP and InGaAs using dimethylzinc as the diffusant source. The resulting diffusion profiles are measured using secondary ion mass spectrometry. Systematic errors introduced by multiple diffusions are explored and a simple model of reactor transients is introduced to explain the observed departures from the expected square-root behavior. Results of zinc diffusion into high-speed avalanche photodetectors are also presented to demonstrate the ability of MOCVD based diffusion to target. diffusion depths with strict tolerances. (C) 2004 American Vacuum Society.