A quasi one-dimensional model in which oblique evaporation wave theory and shock wave theory are employed is developed to investigate the choking phenomenon and the pressure drop mechanism in electronic expansion valves (EEV). The data predicted by the model are validated by the experimental data presented by Zhang. The results show that the choking mass flow rate of EEVs calculated by Abuaf's model, which was developed with water, agrees well with the tested data when R22 is used as the working fluid. The relative deviation between the predicted data and Zhang's test results is in the range from -5% to 6%. Compared with the test data, the choking mass flow rate of EEVs with the mixture R407c is over predicted by Abuars model. The back pressure at the EEV exit when choking just occurs is decreased as the flow area (opening pulse number) is decreased. The trend is consistent with Zhang's test results. (C) 2008 Elsevier Ltd. All rights reserved.