Na2CoP2O7 crystals are synthesized by hydrothermal technique at moderate pressure-temperature conditions. The maximum degree of crystallinity is observed at the critical concentration of initial components, beyond which the crystallinity collapses sharply to a minimum. Solubility studies have revealed positive thermal coefficient of solubility in a H3PO4 solvent with relatively high enthalpy. DTA results revealed this compound to undergo a phase transition at 380 degrees C. (c) 2005 Elsevier B.V. All rights reserved.