Temperature estimates are key to understanding the crystallization of pyrometallurgical slags. To date, temperature approximations have been based mainly on phase diagrams; these give approximate data only. Geothermometers (e.g., phase – whole-rock composition) applied in this study give much more accurate, focused results that are in agreement with historical data. Pyrometallurgical slags related to zinc extraction in Upper Silesia, southern Poland, date back to the 19th century. Thus, there is a great variety of slag textures and compositions reflecting changes in technological processes ranging from the primitive “Silesian method” to more advanced techniques including calcination and precise dosages of coal/coke and add-ons. Slag samples were collected at three localities with known smelting histories: Piekary Śląskie, Katowice and Ruda Śląska. The analysed material is composed of high-temperature phases among which oxides, silicates and aluminosilicates dominate. The chemical composition of the slags is complex, with up to nine major oxide components, namely, SiO2, Al2O3, FeOt, ZnO, PbO, MnO, MgO, CaO and K2O. The three geothermometers applied, i.e., olivine – WR (whole rock), pyroxene – WR and feldspar – WR, yielded temperatures ranging from 987 to 1314°C. Phase diagrams on the other hand, due to the complex chemical compositions involved, were useful for general determinations only; they gave overestimated temperatures in the range 1100–1800°C. Evaluating temperature conditions using geothermometry demonstrated the usefulness of different technological processes at the various sites sampled.