The eddy covariance method is highly recommended for measuring in situ water vapor flux. In this study, eddy covariance systems were installed at two heights (30 m and 100 m) on a micrometeorological tower located in a tropical mixed forest. We have identified a difference in the Priestley-Taylor coefficients (alpha), calculated at 30 m and 100 m, with vertical variation of water vapor flux, and this height-dependent difference is smaller when the vertical difference in wind speeds is higher. It will be useful to consider this result in the analysis of measurement data for model validation. (C) 2013 Elsevier B.V. All rights reserved.