This paper presents a study on the thermal properties of a range of geopolymers in order to assess their suitability for high temperature applications such as thermal barriers, refractories and fire resistant structural members. Geopolymers were synthesised from five different fly ashes using sodium silicate and sodium aluminate solutions to achieve a set range of Si:Al compositional ratios. The thermo-physical, mechanical and microstructural properties of the geopolymers are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. The amount and composition of the amorphous component (glass) of each of the fly ashes was determined by combining XRD and XRF results. It was found that the Si:Al ratio in the glass of the fly ashes strongly influenced the thermal performance of the geopolymers. Geopolymers synthesised from fly ashes with a high Si:Al (≥5) in the glass exhibited compressive strength gains and greater dimensional stability upon exposure to 1000°C, whereas geopolymers synthesised from fly ashes with low Si:Al (<2) in the glass exhibited strength losses and reduced dimensional stability upon high temperature exposure. ► Quantification of the crystalline and amorphous phases of five different fly ashes. ► Si:Al ratio in the glass of the fly ash influences the thermal performance. ► Compressive strength of geopolymers can increase by up to 5 fold after firing. ► Sodium aluminate activated geopolymers exhibit thermal stability up to 800°C. ► Sintering during thermal exposure improved inter-particle bonding.