The thermal-expansion and room-temperature lattice constants were determined for several phosphates and arsenates isomorphous to KH2PO4. From these data, the lattice constants were calculated at the ferroelectric antiferroelectric temperature. They show a nearly identicalc/aratio for the compounds of any given alkali ion and a change ofc/aratio with the ionic radius of the alkali ion. The size of the expansion anomalies at the transition are markedly affected by deuteration. The anomalies in the ammonium compounds are larger by an order of magnitude than those for the ferroelectric alkali compounds. Temperature hysteresis measurements indicate that the transition in KD2PO4is probably second order.