Ultramafic and mafic rocks are possible targets for CO ₂ sequestration via mineral carbonation. The determination of reaction kinetics and the factors that control mineralization are important in order to understand and predict how fast injected CO ₂ will react with host rocks to permanently isolate and store the carbon. Here we present experimental results of olivine carbonation experiments using synthetic fluid inclusions (SFI) as micro-reactors. The micro-reactor technique coupled with non-destructive Raman spectroscopy allows us to monitor the reaction progress in situ and in real time at elevated temperatures (50–200°C) and pressures (several 10's to a few hundred bars), and quantify the amount of CO ₂ consumed in the reaction using the Raman CO ₂ densimeter and mass-balance calculations. Results show a measurable decrease of CO ₂ density in the fluid inclusions as a result of the reaction between the CO ₂ -bearing seawater-like aqueous solution and olivine. Magnesite formation was observed within hours at ≥100°C, while at 50°C magnesite nucleation and precipitation was only observed after a few weeks. Raman mapping and FIB-SEM analysis confirmed the formation of a non-continuous Si-rich layer on the inclusion wall and the presence of ferroan magnesite as a reaction product. Reaction rates [log J (mol/m ⁻² s ⁻¹ )] obtained for olivine carbonation range between ~-8.4 at 50°C and −4.7 at 200°C, which is sufficiently rapid to be suitable for commercial CO ₂ injection projects. Reaction rates involving a seawater-like fluid were similar to rates published for high salinity solutions containing NaHCO ₃ , and were faster compared to rates involving solutions with low salinity. Thus, CO ₂ injection into submarine environments might offer some advantages over CO ₂ storage in onshore basalts where the pores are likely to be filled with low salinity meteoric water. The application of the synthetic fluid inclusion technique, combined with non-destructive analytical techniques, is a promising tool to monitor rates of fluid-rock reactions in situ and in real time. Here, we have documented its application to experimentally study carbonation reactions in the olivine-H ₂ O-CO ₂ -NaCl-MgCl ₂ system.