Impedance spectroscopy was used to measure the changes of electrical properties during the isothermal crystallisation of the phosphate phase in bulk samples of a glass with nominal molar composition (3CaO.P2O5)(20)(SiO2)(35)(MgO)(38.354)(K2O)(6.646), developed for biomedical applications. Crystallization studies were performed in isothermal conditions at 800 degrees C and 860 degrees C, and were monitored by impedance spectroscopy. XRD shows that a stable orthophosphate phase, Ca9MgK(PO4)(7) crystallizes at about 860 degrees C after formation of earlier phosphate precursors formed at lower temperatures, namely oxyapatite (Ca-5(PO4)(3)O) and tricalcium phosphate (Ca-3(PO4)(2)). The conductivity of bulk glass samples decreases at 800 degrees C, as the phosphate precursors phases crystallize. The opposite trend is observed on crystallizing the orthophosphate phase, at 860 degrees C.