Adsorption azeotropy is a phenomenon that has been known for nearly a century, yet few properties have been formally proven. Here four general properties of adsorption azeotropy in porous materials are discussed and shown to apply irrespective of the isotherm, including the fact that there is always a lower bound on the pressure at which an azeotrope may be present. As molecules of different size will favour the occurrence of an azeotrope, this study considers in detail the thermodynamically consistent dual site Langmuir model, where azeotropy is solely the result of the adsorbent heterogeneity. Six categories of adsorption azeotropes, which can be grouped into three pairs of mirror cases, are formally identified for this model. Dimensionless ratios allow to determine formally each category and the analysis also includes a discussion of the crossing of the pure component isotherms. The heterogeneity of the adsorbent is shown to lead to azeotropes that can include an upper bound on pressure; that can occur even if the pure component isotherms do not cross; and can be present below the pressure at which the pure component isotherms cross. Finally, the analysis allows to identify also the ranges of parameters for which the pure component isotherms cross but an azeotrope is not present.