A new diamond ATR FTIR method has been developed to quantify the processes occurring in the resin-modified glass-ionomer cement (RMGIC). Fuji II LC (Improved), at 1 mm depth from the cement/water interface. With Fuji II LC (Improved) various changes in the spectra due to 90% monomer polymerisation were observed within 1 min after 20 s exposure to a dental light. Following polymerisation further different peak shifts with time were detected. Comparison with spectral changes seen during setting of the conventional glass-ionomer cement, Fuji IX, showed these could be assigned to water sorption and/or polyacid neutralisation. Any absorbance change due to the acid/glass reaction alone exhibited 2 linear regions when plotted against square root of time. Such behaviour suggests two separate diffusion mechanisms for acid neutralisation. The first faster one ceases at 30 or 150 min after mixing in Fuji IX and Fuji II LC (Improved), respectively. It was proposed that these were the times at which all the water (a required component of the reaction) in the original formulation is used up. The slower process was the same acid/glass reaction but initiated by water sorption. The initial rates of absorbance change due to acid neutralisation were 17 times faster for Fuji IX than Fuji II LC (Improved). By 4 days however, the total absorbance change due to acid neutralisation for Fuji IX was only 4 times that for Fuji II LC (Improved). Such results can help to explain changes in cement properties with time.