Time-resolved energy dispersive X-ray diffraction (EDXRD) data have been measured in situ from cast blocks of gypsum, CaSO4·2H2O, in the presence of reactive phosphate solutions under hydrothermal conditions (100 ≤T≤ 180 °C) in order to understand the formation of hydroxyapatite monoliths, for applications such as in artificial bone or dental materials. Measurement of data in short (60 s) intervals has thus permitted information about the kinetics and mechanism of transformation of gypsum to hydroxyapatite to be obtained in a non-invasive way, avoiding the irreversible conversion of hydrous intermediate phases that would occur on quenching. At the lower temperatures used gypsum first converts to an amorphous intermediate phase during reaction, but as the temperature is raised to 130 °C and above, hydrothermal dehydration to the subhydrate CaSO4·½H2O always occurs before hydroxyapatite crystallises. The final crystal size of the hydroxyapatite is estimated from the peak broadening of the EDXRD data and this reveals an increase in crystal dimension with increasing reaction temperature. Comparing measurements from the surface and from the core of the blocks shows that an additional phosphate phase, CaHPO4, is observed in the core at two temperatures and also that the crystal growth of hydroxyapatite does not penetrate the core of the block on the time scales we have investigated (up to 6 hours). The observations have important implications in the fabrication of hydroxyapatite monoliths from cast gypsum for applications, since the conversion via several intermediate phases may compromise the integrity and mechanical properties of the monoliths.