In this paper, experiments are performed to determine the convective mass transfer coefficient for evaporation in a horizontal rectangular duct with an aspect ratio of 14.5:1. In the test facility, a short pan of water forms the lower panel of a long duct where a hydrodynamically fully developed laminar or turbulent airflow passes over the surface of the water. The measured convective mass transfer coefficients have uncertainties that are typically less than +/- 10% and are presented for Reynolds numbers (Re) between 570 and 8100, Rayleigh numbers (Ra) between 6300 and 83,000, inverse Graetz numbers (Gz) between 0.003 and 0.04, and operating conditions factors (H*) between -3.6 and -1.4. The measured convective mass transfer coefficients are found to increase as Re, Ra, Gz and H* increase and these effects are included in the Sherwood number correlations presented in this paper, which summarize the experimental data. (c) 2006 Elsevier Ltd. All rights reserved.