AbstractThe role of water of hydration on the crystal structure, the solubility and the rate of dissolution of fenoprofen calcium dihydrate was examined. The rate of dehydration of fenoprofen calcium dihydrate at 0% relative humidity (R.H.) increased from 0.0400 to 0.7488 fraction dehydratedhour over a temperature range of 50°C to 80°C and appeared to occur by a combination of diffusion and nucleation processes. The enthalpy of dehydration was 21.897 kcal/mole. Hydration of dehydrated fenoprofen calcium occurred rapidly at 100% R.H. X-ray powder diffraction analysis showed that the upon dehydration fenoprofen calcium dihydrate underwent a change in its crystal structure to form anhydrous fenoprofen calcium. The rate of dissolution of fenoprofen calcium dihydrate tablets at 37°C was not significantly different than that of tablets containing anhydrous fenoprofen. The enthalpies of solution (δHsol) of fenoprofen calcium dihydrate and anhydrous fenoprofen calcium were 2.0504 and 3.5583 kcaYmole respectively. The transition temperature at which the dihydrate and anhydrous fenoprofen had equal solubilities was approximately 59.91°C.