Crystals of CaK2As2O7and CdK2P2O7, both grown from the melt, are monoclinic withZ = 4. The lattice parameters area = 9.222(6),b = 5.835(3),c = 14.698(10) Å, β = 105.84(5)° with space groupP21/cfor the diarsenate anda = 9.737(2),b = 5.548(1),c = 12.766(2) Å, β = 106.50(2)° with space groupC2/cfor the diphosphate. The structures were refined by full-matrix least-squares methods utilizing 2070 reflections (R = 0.056) for the diarsenate and 1145 reflections (R = 0.067) for the diphosphate. Both structures contain pseudo-hexagonally packed anions, in staggered configurations, forming layers with the divalent cations in six coordinate sites between the layers. The average M—O bond lengths are 2.342 and 2.290 Å for M = Ca and Cd respectively. The K ion has nine oxygen atoms with mean K—O bond lengths of 2.943 and 3.020 Å in the diarsenate in the coordination sphere. The mean of the ten shortest K—O is 2.939 Å in the diphosphate.