In this work, the reaction mechanism used in the preparation of fluorine-free superconducting YBa2Cu3O7-delta (YBCO) was investigated. To determine which precursor interactions are dominant, a comprehensive thermal analysis (thermogravimetric analysis-differential thermal analysis) study was performed. The results suggest that a three step reaction mechanism, with a predominant role for BaCO3, is responsible for the conversion of the initial state to the superconducting phase. In the presence of CuO, the decarboxylation of BaCO3 is kinetically favored with the formation of BaCuO2 as a result. BaCuO2 reacts with the remaining CuO to form a liquid which ultimately reacts with Y2O3 in a last step to form YBCO. High temperature X-ray diffraction experiments confirm that these results are applicable for thin film synthesis prepared from an aqueous fluorine-free sol gel precursor.