Calculations based on simple interaction potentials are performed to define the adsorption characteristics of molecules encapsulated in carbon nanotubules. The continuum approximation used to describe the cylindrical sheets is shown to work fairly well within a large range of tubule diameters. Criteria for molecule confinement are given which include the influence of the number of graphitic shells describing the whole tubule and they are compared to similar results obtained for carbon Buckyballs. Application to the most common encaged species, i.e., rare gas atoms and alkali-metal ions, confirms these general rules.