The presence of "exotic" scale such as Zinc Sulfide (ZnS), Lead Sulfide (PbS) and Iron Sulfide (FeS) in HP/HT reservoirs has been identified. "Exotic" scale materials come as a new challenge in HP/HT reservoirs. This has led to the development of more advanced tools to predict their behavior at extreme conditions. The aim of this work is to include ZnS into the group of scale materials that can be modeled with the Extended UNIQUAC model. Solubility data for ZnS are scarce in the open literature. In order to improve the available data, we study the experimental behavior of ZnS solubility at high temperatures. The determination of the solubility of ZnS is carried out at temperatures up to 250°C. Zinc sulfide (99.99%) and ultra-pure water are placed in a vial in a reduced oxygen atmosphere. The sample is placed in a controlled bath and stirred until equilibrium is attained. The suspension is filtered at the same process temperature and diluted immediately. Afterwards the aqueous solution is analyzed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) as analytical technique. The concentrations of Zn 2+ and S 2- ions are analyzed. The experimental data are used for parameter estimation in the Extended UNIQUAC model presented by Villafafila et al. (2005); (2006). The solid-liquid phase equilibria of the system is represented using the Extended UNIQUAC model. This model is chosen due to its versatility and fewer number of parameters (two parameters per species plus two parameters per species pair) to be estimated compared to other models (Pitzer's model). It is observed that ZnS solubility increases with increasing the temperature. This temperature dependency is very well represented by the Extended UNIQUAC model. The model is capable of predicting experimental data within the experimental error. The agreement between experimental data and the Extended UNIQUAC model shows that this thermodynamic model is a promising tool capable of determining the occurrence of ZnS scaling in HP/HT reservoirs. This methodology can be extended to other scaling materials (PbS, FeS), making the Extended UNIQUAC model a leading model in predicting scaling in HP/HT reservoirs.