Activated alumina, for application as a catalyst support or as an adsorbent, is often obtained industrially via calcination of precipitated aluminum oxide hydrate. Its preparation conditions depend on the ultimate application, which, in turn, dictates its target specifications, e.g., surface area (SA), total pore volume (PV), pore size distribution (PSD) and density. Invariably, some method of porosity characterization is required periodically during manufacture for process control and at the final stage of alumina certification. Although gas and vapor adsorption methods were common in the early days of alumina production, mercury porosimetry (NIP) has become the technique of choice in the past 30 years. MP provides valuable information about the meso- and macro-pore structure of the alumina but the hysteresis, seen in each analysis, has been difficult to interpret. In this work the NIP porosity of a wide variety of calcined aluminas is contrasted with thermogravimetric (TGA) analyses of the source aluminum oxide hydrates and the precipitation and activation conditions, which generated the aluminas in an attempt to better understand the hysteresis phenomenon. The results imply a "connectivity", or internal structure, explanation for MP hysteresis in calcined aluminas.