The reaction kinetics and mechanism of geopolymers are studied. The dissolved silicate concentration decreases from the beginning of the reaction. A characteristic time 't(0,vit)' for the setting of the reaction mixture is derived from isothermal Dynamic Mechanical Analysis experiments. 't(0,vit)' increases with SiO2/R2O but goes through a minimum for increasing water content. The reaction is slower for K compared to Na-silicate based systems. Si-29 and Al-27 solution NMR are used to probe the molecular changes. Al-27 NMR and FTIR reveal that an 'intermediate aluminosilicate species' (IAS) is formed from the start of the reaction. The concentration decrease of OH- during low-temperature reaction is related to the formation of IAS. The rate law of this process seems to be obeyed by a total reaction order of 5/3, with a partial order of 1 for OH- and 0 for Na(+)in the silicate solution. During first heating after polymerization water is lost leading to a distortion of the Al environment. According to XRD, no crystallization occurs below 900 degrees C. However, between 950 and 1100 degrees C a crystallization exotherm of nepheline is observed with DSC for a geopolymer with SiO2/Na2O = 1.4. Neither T-g of the amorphous geopolymer, nor the shrinkage and expansion around T-g during first heating, cause a measurable heat effect.