The properties of the product obtained through alkaline activation of metakaolin are directly influenced by the characteristics of the starting kaolin. For the study of this influence, a complete characterization of two spanish kaolins used to synthesised the material has been carried out. A JAF conduction calorimeter was used to follow metakaolin reaction with NaOH solutions. Calorimetric data were obtained isothermally at 45 degreesC with variable solution/solid ratio and NaOH solution concentrations varying from 12 to 18 M.