The reactivity of solid fuels i.e. fossil fuels, waste fuels and coal processing products is a feature decisive for the reaction rate under various burning conditions and affects both the bum-up factor of the fuel in the furnace and hence also the combustion efficiency, which further influences the usability of the so-called furnace wastes (too high content of underburnts excludes further utilisation), the ability of fuels to create NOx, temperature distribution in the furnace etc. The reactivity of solid fuels is not actually tested on the standardised basis because of the lack of unified definitions of terms, deficiency of a fairly simple laboratory method for the reactivity determination and of a practical method for the interpretation of the test results. A step towards standardisation of the methodology of reactivity tests can be made by using such terms as combustion intensity it [kg/s], surface combustion rate q [kg/m(2)/s] and chemical reaction rate constant K-kin, [kg(C)/m(2)/s/Pa-0,Pa-5] together with kinetic constants k and E determining the fuel reactivity.