Lithium oxosilicate was synthesized via the solid-state method using Li2O and SiO2 as starting reactants. In situ synchrotron powder X-ray diffraction (SPRXD) coupled with Rietveld refinement allowed describing the synthesis as a two-step process where Li2O and SiO2 react to form Li4SiO4 and, at higher temperatures, lithium orthosilicate reacts with the remaining Li2O to form Li8SiO6. Time-resolved measurements allowed determining the temperatures at which each phase transformation occurs as well as the time required to complete the synthesis. The CO2 capture properties of Li8SiO6 in the temperature range from room temperature to 770 °C were studied in detail by time-resolved in situ SPXRD. The crystallographic phases present during Li8SiO6 carbonation were identified and quantified via Rietveld analysis. Results showed that, within the temperature range from 200 to 690 °C, Li8SiO6 carbonation produces Li4SiO4 and Li2CO3, while, at temperatures from 690 to 750 °C, a secondary reaction occurs, where previously formed Li4SiO4 reacts with CO2, producing Li2SiO3 and Li2CO3. These findings allowed proposing a mechanism of reaction for Li8SiO6 carbonation in the temperature range that is of interest for high temperature solid-state sorbents.