John (J) isotherm model stated as log log P = C + n logV, for solid-gas equilibrium, is a unique contribution from India for the study of porous materials. These isotherms are generally characterised by different phases of adsorption marked by a sudden change in slope and sharp kinks in isotherm plots of loglog P versus log V. John isotherm is otherwise known as the phase change method. The isotherm model envisages the degree of porosity, by which the categorisation of porous materials could be done. The volume adsorbed, V(J) corresponding to saturation pressure P-s is taken as the limiting micropore volume (LMV) or John adsorption capacity. The adsorption behaviour of some known microporous carbons has been analysed using John isotherm. John isotherm along with five other isotherm models, Freundlich, Langmuir, D-R, BET and I plot methods are studied, constants and parameters are compared. There are three carbons, namely GC, its hydrogen treated form H2TGC and the nitric acid treated prodigy NITGC, whose isotherm data available is used to plot John isotherms to report the merits of the isotherm method. The results reveal that John isotherm model give excellent fit to the reported experimental data and provide precise information about the funcional mechanism of adsorption. The study aims to establish the application of John isotherm as a simple empirical isotherm model for characterising the microporosity of carbon materials over a wide range of concentration and relative pressure. The main purpose of this research paper is to reaffirm the application of John isotherm for its universal acceptance to study materials of microporous nature.