The effects of metal and alkali promoters on precipitated iron oxide Fischer-Tropsch catalyst precursors are examined using thermal gravimetry (TG) and differential thermal analysis (DTA). A distinct exotherm corresponding to what appears to be the transformation from FeOOH to Fe2O3 is observed for unpromoted and promoted iron oxides when the promoter’s ionic radius is less than approximately 0.7 Angstrom. The heat released during the exotherm is similar for unpromoted and promoted iron oxides corresponding to this size range. For promoters having ionic radii larger than 0.7 Angstrom, the exotherm occurs over a broad temperature range. Silica exhibits a unique propensity to stabilize against this transition, and appears to be present as two species. Increased loadings of Si cause a shift from the sharp, narrow temperature range exotherm to the broad transition observed for promoters with larger ionic radii.