The study of direct contact condensation (DCC) of steam is important owing to the diverse range of DCC applications, including uses in power plants, heat exchangers, and steam ejectors. In this study, an experimental investigation was performed to study the thermal hydraulic characteristics of the DCC of steam in an unstable condensation regime. Deionized water was used as the working fluid obtained with the use of a reverse osmosis method. The experiments were performed with pool temperatures in the range of 70 to 92 degrees C +/- 2 degrees C, with different steam mixture mass fluxes in the range of 5 and 64 kg/m(2) s, and with a gas injection nozzle with an inner diameter of 9.525 mm. The pool temperature was measured using thermocouples, and the surface area of the bubble and the volume flow rate were evaluated with high-speed camera image acquisitions. Subsequently, image processing was used to obtain the heat-transfer coefficient and other thermal hydraulic characteristics, such as the bubble condensing frequency and the bubble centroid velocity. The calculated condensation heat-transfer coefficients were found to be 24.57-179.78 kW/m(2).K. According to the experimental results at various boundary conditions, a unified correlation of the heat-transfer coefficient was identified at the bubbling frequency in the unstable condensation regime, regardless of the pool subcooling temperature and steam mass flux. It was also found that the characteristic frequency of the steam bubble can be determined based on the boundary conditions to allow the determination of the heat-transfer coefficient with the given boundary conditions. (C) 2019 Elsevier Ltd. All rights reserved.