A series of solid solution phases between ettringite, Ca6Al2(SO4)3(OH)12·26H2O, and thaumasite, Ca3SiSO4CO3(OH)6·12H2O, have been prepared and analysed by X-ray powder diffraction and full pattern fitting. Solid solutions were shown to exist with both the ettringite structure (space group P31c) and the thaumasite structure (space group P63, c-axis halved). A possible discontinuity was identified, characterised by a gap in the a-dimension of the solid solution phases produced. This discontinuity is believed to correspond to a switch between the ettringite space group and the thaumasite space group. It is suggested that any discontinuity in the solid solution is caused by differences in the hydrogen bonding of the two structures.