1. The cyclization of isoprenoid compounds under the influence of H2SO4 can be accompanied by the formation of sulfuric acid esters, the secondary transformations of which complicate the general course of the reaction. 2. Fluorosulfonic acid, which is incapable of forming esters and for this reason assures a high “degree of development” of the carbcation center of the molecules, offers promise as a reagent for the cyclization of isoprenoids. 3. Isoprenoid compounds, containing different functional groups that are close in character (−OCOCH3, >CO, −COOR), exhibit at low temperatures (−70°) a variable capacity to complex with the proton of an acid, which has an important effect on the cyclization rate of these compounds.