In situ energy-dispersive X-ray diffractometry, using a polychromatic synchrotron beam and a 'laboratory-sized' sample, is used to provide a direct measurement of the kinetics of geopolymerisation. The effects of sample SiO2/Al2O3 ratio, Na/(Na+K) ratio and reaction temperature are investigated. The results obtained support recent propositions that the initial gel phase formed during geopolymerisation is later transformed to a second, probably more-ordered gel phase, and provide detailed information regarding the rate of formation of the first gel phase during the first 3 h of reaction. Increasing the SiO2/Al2O3 ratio generally decreases the initial rate of reaction, with the highest SiO2/Al2O3 ratio samples showing what appears to be a pause in the reaction corresponding roughly to the solidification of the geopolymeric binder. Mixed (Na,K)-aluminosilicate geopolymers with moderate SiO2/Al2O3 ratios behave similarly to pure Na- or K-aluminosilicate compositions of higher SiO2/Al2O3 ratio. Fitting a simplified first-order rate expression to the overall reaction process at different temperatures allows the calculation of an effective overall activation energy, which may be useful in comparing geopolymerisation of slurries with different compositions. (C) 2007 Elsevier Ltd. All rights reserved.