Borosilicate glass is an important material used in various industries due to its chemical durability, such as for the immobilization of high-level nuclear waste. However, it is susceptible to aqueous corrosion, recognizable by the formation of surface alteration layers (SALs). Here, we report in situ fluid-cell Raman spectroscopic experiments providing real-time insights into reaction and transport processes during the aqueous corrosion of a borosilicate glass. The formation of a several-micrometre-thick water-rich zone between the SAL and the glass, interpreted as an interface solution, is detected, as well as pH gradients at the glass surface and within the SAL. By replacing the solution with a deuterated solution, it is observed that water transport through the SAL is not rate-limiting. The data support an interface-coupled dissolution-reprecipitation process for SAL formation. Fluid-cell Raman spectroscopic experiments open up new avenues for studying solid-water reactions, with the ability to in situ trace specific sub-processes in real time by using stable isotopes.