Greek lignite samples with various ash qualities were gasified after drying, without any other treatment, in H2 and CO2 atmospheres, in order to study mineral matter effects on lignite gasification. Tests were performed in a fixed bed reactor operating at ambient pressure. Reaction rate constants (k), in both hydrogasification and carbon dioxide gasification were calculated from Arrhenious plots. The alkali index (AI), was evaluated for each lignite sample in an effort to correlate it with the obtained results. In the studied region of alkali indices, the reaction rate constant (k) varied almost linearly with the alkali index, for both H2 and CO2 gasification. Gasification rate increased proportionately to Ca concentration, but such correlation was not evident for Fe, possibly due to its chemical form in the lignite. The gasification rate showed an initial increase and a subsequent decrease with Mg and Al concentrations, for both H2 and CO2 gasification. The inorganic constituents in Greek lignite seem to play a controlling role in determining gasification reactivity. A reasonably good correlation exists between the gasification rate and the alkali index.