This paper addresses the role of accessibility for adsorption in porous solids on the adsorption properties including Henry constant, adsorption isotherms and isosteric heat of adsorption. The relevant parameters are the accessible volume, the accessible geometrical surface area and the accessible pore size and its associated volume. This concept will be demonstrated to be important and calls for the need to consider adsorption characteristics in the most coherent and consistent manner. It is particularly reinforced by the limitations inherent in the conventional ways in determining the void volume, surface area and pore size. We provide a number of examples to support this; the challenge that faces us is the development of consistent experimental procedures to determine these accessible quantities. We define the accessible pore size as the size of the largest sphere that rests on three closest solid atoms in such a manner that any probe particle residing in that sphere would have a non-positive solid-fluid potential energy. For each accessible pore size there is an associated accessible pore volume, giving rise to a new accessible pore size distribution (APSD). This is distinct from the classical pore size distribution commonly used in the literature, and in our definition of accessible pore size, a zero pore size is possible. It is also emphasized that the accessible quantities that we introduce here are dependent on the choice of molecular probe, which is entirely consistent with the concept of molecular sieving.