A transient heat balance model (THBM) based on energy conservation is developed for predicting the thermal performance of a semi-transparent water wall system. In order to validate the THBM against field measurements, real climate conditions in Sydney, Australia are adopted and the time variations of both internal and external convective and radiative heat transfer coefficients are calculated using empirical correlations from the literature. An alternative concrete wall model is also developed for further validation of the THBM. Good agreements between the THBM simulation and field data are achieved for both the water wall and concrete wall systems. The validated THBM is then adopted in a sensitivity analysis in order to assess the thermal performance of the semi-transparent water wall system under different configurations. It is found that the daily maximum water and air temperatures decrease significantly with the decrease of the transmissivity of the semi-transparent panels, whereas an increase of the attenuation coefficient of solar radiation in water has an insignificant impact on the water and air temperatures. The present results also indicated that increasing the thickness of the water column can reduce the fluctuations of the water temperature and the diurnal peak air temperature. From the practical application point of view, reducing the transmissivity of the external panel provides the most effective way to mitigate over-heating in the semi-transparent water wall system. (C) 2016 Elsevier Ltd. All rights reserved.