By application of the modified BET isotherm on adsorption data, for adsorption isotherms which are neither purely of the I or IV IUPAC type, it is possible to determine simultaneously the net micropore volume, the monolayer capacity and the BET parameter C. Specific surface obtained from the monolayer capacity represents the mesopore surface area and is not overestimated as in the case when the presence of micropores is neglected. The obtained BET parameter C can serve for choosing the appropriate master isotherm which is required for calculation of the mesopore-size distribution. The net micropore volume characterises the adsorption capacity when the adsorption mechanism is volume filling of pores.