In this paper, we develop a theory to investigate the structural heterogeneity in the adsorption equilibria for gas-solid systems. The structural heterogeneity is characterized by the micropore size distribution. We assume that the solid is made up of patches of pores having the same size and that the local isotherm at a given patch follows either the Langmuir or Jovanovic equation. The network of patched micropores follows a certain distribution. We collected adsorption equilibrium data for a range of adsorbates, ethane, propane, butane, pentane, benzene and carbon dioxide, on an Ajax activated carbon. With these data, and data available from the literature on a number of activated carbons, we extracted their micropore size distributions with the constraint that these distributions must be independent of adsorbate and temperature.