Abstract In-situ synchrotron X-ray reflectivity (SXR) measurements of the calcite-water interface were made. These data were used to model the angstrom-scale interfacial width and electron density profile of the interface. Measurements were made on the (10T4) cleavage surface of Iceland spar calcite in contact with pure N 2, humid N 2, deionized water, and dilute nitric acid solutions at 25°C. The measurements were performed in an X-ray transparent reaction cell in which fluid pH was monitored during SXR data acquisition. Analysis of the data indicated that the pristine calcite surface in contact with pure N 2 has an interfacial width (statistical roughness) of 2.60 ± 0.15 Å. Exposure to water vapor (humid N 2) resulted in the deposition of a water film 19.9 ± 0.4 Å thick. There was no evidence from the SXR data that the adsorbed water film caused roughening of the calcite surface. However, when reacted with deionized water the calcite was roughened, yielding an interfacial width of 23.1 Å. During successive reactions with fluids having decreasing initial pH the calcite/fluid interfacial width increased from 23.1 to 45.3 Å. The SXR results indicate that intraterrace areas participate in the dissolution of calcite. A linear correlation was observed between interfacial width of the intraterrace areas and the quantity of calcite dissolved.