Amorphous phase separation of ionomer glasses, also known as fluoro-phospho-alumino-silicate glasses, of generic composition SiO2-Al2O3-P2O5-CaO-CaF2 were investigated. A sodium-boro-silicate glass system, which is known to undergo amorphous phase separation was also investigated. High Temperature Dynamic-Mechanical Thermal Analysis ( DMTA), combined Differential Thermal Analysis/Thermo Gravimetric Analysis (DTA/TGA) and Scanning Electron Microscopy (SEM) were the principal analytical techniques used in this study. High temperature DMTA was used to measure the glass transition temperatures (T-g) of the original starting glass compositions, as well as being able to follow amorphous phase separation (APS) within the glass. High temperature DMTA traces of both the ionomer glasses and the sodium-boro-silicate glasses exhibited two maxima in tan delta, corresponding to two glass transition temperatures and demonstrating that amorphous phase separation of the parent glass into two glass phases had occurred. DTA/TGA of the ionomer glasses detected a glass transition and two crystallisation peaks for apatite and mullite, accompanied by a gradual weight loss of 1 - 3% on passing through the crystallisation region. The sodium-boro-silicate base glass showed no evidence of a glass transition, but a prominent glass transition was detected for a second sample which had undergone a heat-treatment of 240 min at 580degreesC. SEM analysis of the ionomer glass compositions revealed smooth spherical droplets of 2 - 15 nm while the background morphology appeared rough and speckled. A classic interconnected structure was observed for the sodium-boro-silicate glass. (C) 2003 Kluwer Academic Publishers.