The crystal structure of triclinic potassium dichromate has been determined by single crystal X-ray diffraction. The cell constants are a = 13.367, b = 7.376, c = 7.445 Å, α = 90.75°, β = 96.21°, γ = 97.96° with four K2Cr2O7 units per cell. Refinement of 2600 reflections in both the space groups P1 and [Formula: see text] leads to the same structure. This is in agreement with the results of anomalous dispersion measurements, confirming that [Formula: see text] is the correct space group. The final agreement index, R, is 0.054. The two crystallographically independent dichromate ions are similar, deviating only slightly from C2v) symmetry. The Cr—O (terminal) distance is 1.63 Å, the Cr—O (bridging) distance is 1.79 Å and all angles at the chromium atoms are tetrahedral except for one of the O(bridging)—Cr—O(terminal) angles in each ion which is 106°. The angles at the bridging oxygen atoms are 124° and 128°. The geometry of the anion is compared with that found in a number of similar groups.