Results of solubility determinations on naturally-occurring calcium silicate hdrates are reported for okenite, gyrolite, tobermorite and xonotlite: also for phase pure laboratory preparations of tobermorite, afwillite, foshagite and hillebrandite. Samples, typically 100-200 mg, were contained in sealed reactors and dispersed in initially pure, CO2-free waterw and equiliberated for up to 110 davs. The pH, calcium and silicon solubilities are reported at 25degrees, 55degrees and 85degreesC. Steady state solubilities are attained in 7 days at 25degreesC but more rapidly at 55degrees and 85degreesC. Recommended Ksp values are tabulated. Sources oferror are discussed: shortage of samples required that the same mineral had to be redispersed sequentially in flesh leachant and this may have effected the surface state of incongruently soluble phases. Natural minerals may be unreliable in the context of solubility, determination owing to their large content of non-essential ions, for example Na+, K+ and Mg2+.