The combustion of alternative fuels in direct contact with the bed material of the rotary kiln may cause local reducing conditions and, subsequently, decomposition of sulfates from cement raw materials, increasing the SO2 concentration in the gas phase. The decomposition of sulfates increases the sulfur circulation and may be problematic because high sulfur circulation can cause sticky material buildup, affecting the process operation of the cement kiln system. The SO2 release from cement raw materials during combustion of pine wood and tire rubber has been studied experimentally in a high-temperature rotary drum, focusing on the influence of the fuel particle size and volatile content. The SO2 release increased with a decreasing fuel particle size and with an increasing fuel volatile content. Furthermore, CO, H2, and CH4, which are the main reducing gases released during fuel devolatilization, were introduced in different concentrations under the bed material. A threshold concentration for each reducing gas, below which no SO2 release occurs, was found. Introduction of the same molar amount of gas in different concentrations during different time periods showed that a higher reducing gas concentration during a short period (representing fuel devolatilization) released a higher total SO2 amount compared to a lower concentration during a long period (representing fuel char oxidation).