Carbonation of synthesized calcium-silicate-hydrate (C-S-H) is difficult to avoid and can have significant impact on the molecular structure. Considerable carbonation was observed in C-S-H synthesized from the double decomposition of sodium silicate and calcium nitrate solutions but not in C-S-H synthesized from the direct reaction of fumed silica and calcium hydroxide solution. In order to isolate the cause of the greater carbonation in C-S-H synthesized by double decomposition, carbonation was induced in phase-pure C-S-H by reaction with four different water-based solutions containing dissolved CO2 with varying pH and alkali content. Powder X-ray diffraction, thermogravimetric analysis, and Si-29 nuclear magnetic resonance were used to probe the carbonation and the resulting changes in molecular structure. The pH of the solution was seen to strongly influence the degree of carbonation, while the alkali content had much less effect.