In this work, high-temperature-resistant phosphate molecules are applied to characterize ultrathin (100 nm) calcium silicate (C-S) phases. These C-S phases are synthesized on silicon wafers, and the interaction of phosphates with the C-S phases is studied by means of in situ transmission Fourier transform infrared (FTIR) spectroscopy. At room temperature, the chemistry of the system is dominated by the formation of calcium phosphates (C-P). In the case of temperature rising to 1000 degrees C, the C-S phases are regenerated. FTIR results are analyzed on the basis of first principles calculations and further supported by complementary time-of flight secondary ion mass spectrometry (ToF-SIMS) experiments. This study provides a detailed and self-consistent picture of the chemical and structural properties of interfaces such as the one between the atmosphere and ultrathin C-S phases (gas/C-S) and the one between them and silicon wafers (C-S/Si bulk). The material combination of ultrathin-C-S phases grown on silicon wafers might in the future have great potential in selective chemistry, catalysis, and sensing technology as well as in semiconductor manufacturing.