Slag is one of the main waste materials of the iron and steel manufacturing. Every year about 20 × 106 tons of slag are generated in the U.S. and 43.5 × 106 tons in Europe. The valorization of this by-product as heat storage material in thermal energy storage (TES) systems has numerous advantages which include the possibility to extend the working temperature range up to 1000 °C, the reduction of the system cost, and at the same time, the decrease of the quantity of waste in the iron and steel industry. In this paper, two different electric arc furnace (EAF) slags from two companies located in the Basque Country (Spain) are studied. Their thermal stability and compatibility in direct contact with the most common heat transfer fluids (HTFs) used in the concentrated solar power (CSP) plants are analyzed. The experiments have been designed in order to cover a wide range of temperature up to the maximum operation temperature of 1000 °C corresponding to the future generation of CSP plants. In particular, three different fluids have been studied: synthetic oil (Syltherm 800®) at 400 °C, molten salt (Solar Salt) at 500 °C, and air at 1000 °C. In addition, a complete characterization of the studied slags and fluids used in the experiments is presented showing the behavior of these materials after 500 hr laboratory-tests.