The article is devoted to the early period of musumification one of the most significant monuments of St. Petersburg –
St. Isaac’s Cathedral, built in 1818–1858 by architect A. Montferrand. After the October Revolution this cathedral was used
for religious services. The services were held there until 1928, however, since the mid-1920s public authorities are beginning
to attempt museumification of cathedral. On the basis of documents from few archives of St. Petersburg, the author recovers
the history of this transformation of the cathedral into a museum. The community of believers was to monitor the status of
the cathedral and to conduct its own expense restoration work. It could not do it, and by the mid-1920s cathedral was in poor
condition. This was the reason to begin the process museumification. The first exhibition plan for the cathedral was designed
already in 1928. He assumed the holding in the cathedral exhibition dedicated to the history of its construction and interior
decoration. The academic nature of the exhibition has caused a number of complaints on the part of government officials.
But the exhibits brought to the cathedral from other institutions, became the basis for the creation of historical exposition
here later.