Nitrogen sorption at 77 K is a standard technique for the characterization of mesoporosity, However, it seemed to be unable to detect the full, theoretically expected pore volume in high-porosity aerogels, Investigating this problem, we found that if only the sample is sufficiently equilibrated upon sorption, the full pore volume is detected regardless of its porosity. As insufficient equilibration also effects the pore size distribution deduced from the isotherm, highly erroneous results can be avoided if complete equilibration upon each sorption step is assured. For compliant aerogels the effect of sample deformation during analysis has to be taken into account.