Our report presents the enormous work of codicologist fathers B. Sargisyan, G. Sarkisian, S. Chemchemyan and their assistants at St. Lazarus: that huge work is the scientific description of manuscrpits in the depository of the Venice Mechitarists and the compiling of manuscripts' catalogues. The catalogue consists of 8 volumes. We also describe the catalogue of Constantinople’s Lusavorich Church manuscripts, compiled by Arsen Ghazikyan. The catalogue is the only evidence of these lost manuscripts today. We examine the principles of the catalogues’ structure and their features, comparing them with the accepted orders and today’s principles.