There is a growing research interest in CO2 mineral sequestration methods that follow an intermediate Mg extraction step (from Mg-silicates, especially serpentinite rock) by fluxing with ammonium sulfate (AS) or ammonium bisulfate (ABS). This study reports the use of thermogravimetry (TG) combined with differential scanning calorimetry (DSC), mass spectrometry (MS) and/or Fourier-transform infrared spectrometry (FTIR), to explore the serpentinite/flux [(S)/AS and S/ABS] reaction chemistry in more detail and identify conditions under which flux losses are restricted. TG-DSC-MS results show that AS decomposition proceeds through a series of reactions leading to the formation of ammonium pyrosulfate [(NH4)2S2O7, APS] via an ABS intermediate. That APS is the key intermediate is attested by the fact that the analogous potassium salt is a well-known flux for metal oxides. As expected the mechanisms for S/AS reaction are more complex than those of thermal decomposition of pure AS or ABS compounds. Two likely possibilities were identified with S/AS thermolysis: formation of APS or sulfamic acid (SA) precursors that extract Mg/Fe cations from serpentinite above 400 °C. A sulfur dioxide peak was detected on the ensemble spectra at 280 °C. This indicates a loss of ABS through sublimation rather than a complete degradation of AS or ABS reagents. At a fast heating rate of 40 K min−1, tests on S/AS resulted in a significantly lower weight loss (ΔW) than at 10 K min−1 (46% vs. 54%), implying better retention of flux and higher extraction efficiency. From TG-FTIR tests, the presence of humidity has a suppressive effect on SA volatilization, stabilizing the hydrated intermediate APS and/or ABS. It also inhibits mineral transformation to the less reactive forsterite (Mg2SiO4). Extraction of magnesium is primarily dependent on serpentine particle size, but it can be increased significantly in the presence of humidity.