Calcium K-edge (4038 eV) X-ray absorption spectroscopy (XAS) of wet and dry Ca-silicate hydrate (C—S—H) samples and model compounds (calcite, xonotlite, 11 Å tobermorite, and 14 Å tobermorite) show that the local structure of single-phase C—S—H as viewed from the Ca-positions is similar to that of tobermorite. The only major effect of wetting and drying observed by Ca-XAS is an increase in the average Ca-Ca distance, probably due to changes in interlayer spacing. The Ca-XAS spectrum of a fully hydrated β-C2S sample, containing C—S—H and portlandite, is significantly different due to the presence of the portlandite. Ca-XAS provides unique information about the structure of C—S—H and is likely to be a useful tool for investigating the structures of a wide range of phases of importance to cement science.