A study of commensurate transitions in a lattice model of adsorbed straight chain alkanes (ethane, hexane and heptane) in the zeolite Silicilate is presented, and a comparison made of the calculated isotherms with those from experiments and Monte Carlo simulations. The unusual isotherm shapes predicted by the model are in broad agreement with those from Monte Carlo simulations and experiments; thereby supporting the interpretation of such inflections in isotherms first put forward by Smit and Maesen.