  In this work we study the multilayer adsorption of polyatomic species on
homogeneous and heterogeneous bivariate surfaces. A new approximate analytic
isotherm is obtained and validated by comparing with Monte Carlo simulation.
Then, we use the well-known Brunauer-Emmet-Teller's (BET) approach to analyze
these isotherms and to estimate the monolayer volume, $v_\mathrm{m}$. The
results show that the value of the $v_\mathrm{m}$ obtained in this way depends
strongly on adsorbate size and surface topography. In all cases, we find that
the use of the BET equation leads to an underestimate of the true monolayer
capacity.
