A simple vapor transport model for a high-density protein crystal growth (HDPCG) microgravity vapor diffusion apparatus was developed to estimate the evaporation rate of a crystallizing solution including protein and precipitant. The independent variables were the precipitant concentrations and the solution volume. A series of experiments using aqueous sodium chloride with various initial conditions was carried out to verify the model. It was found that the experimental results were well reproduced by the theory. A method was introduced to estimate the parameter, k(VP), which relates the precipitant concentration in the solution to the water vapor concentration in the air. For aqueous NaCl the determined value of k(VP) value coincided well with the known value. In addition, k(VP) values of some other precipitant systems were estimated, and tabulated. (C) 2003 Elsevier B.V. All rights reserved.