A mechanistic model has been developed for the prediction of sieve tray point efficiency for both aqueous and hydrocarbon systems. A forerunner of the model was developed from fundamental relationships by Prado and Fair (Ind. Eng. Chem. Res. 1990, 29, 1031) and was based on gas-liquid transfer in systems predominantly air and water. The earlier model has now been modified and extended. Contacting on a sieve tray is considered analogous to gas-liquid contacting in a mechanically agitated vessel, and supporting research has been adapted to the tray-contacting case. Studies in bubble columns have provided information on bubble size distribution and bubble stability at high pressure; these studies coupled with isotropic flow theory have formed the basis for correlating bubble size distribution as well as the fraction of small bubbles in the sieve tray froth. New tray efficiency data taken on a semi-industrial scale have been combined with published data as well as new data released by Fractionation Research, Inc. to form a large database suitable for model testing. This work is covered in part 1. In part 2, details of the new model will be presented along with comparisons of predicted and measured efficiencies. Predicted efficiencies were found to be within +/-25% of the measured (or deduced) values of the same efficiency. Comparisons are included showing the fit of the same data bank to the earlier model of Chan and Fair.