Summary: Tacharanite has been re-examined using electron microscopy and diffraction, analytical microscopy, X-ray powder and fibre rotation photographs, and other methods. The composition approximates to Ca₁₂Al₂Si₁₈O₆₉H₃₆, and the X-ray diffraction patterns can be referred to an A-centred monoclinic pseudo-cell with a 17·07, b 3·65, c 27·9 Å, β 114·1°, Z = I. In the true cell b is certainly, and a and c probably, doubled. Small, reversible changes in pseudo-cell parameters occur on heating below 200 °C, and parameters found by electron diffraction differ slightly from those found with X-rays, presumably due to shrinkage on dehydration in the high vacuum of the electron microscope. An earlier report that tacharanite changes into a mixture of tobermorite and gyrolite on standing in air is not confirmed. Tacharanite shows some important resemblances to tobermorite, but there are also significant differences.