[en] The research in the area of graphene derivatives containing chalcogen elements is currently an exclusive domain of oxygen- and sulfur-containing functional groups. Here we report an effective introduction of selenium-based organic functional groups and propose a mechanism of their attachment to graphene and graphene-related materials. Fluorographene can be effectively functionalized by selenium-containing group, forming C-Se bonds without undesired formation of elemental selenium, using selenourea. A similar reaction on graphene oxide is accompanied by the formation of elemental selenium due to oxidation of selenourea by oxygen-containing functional groups of graphene oxide. The formation of C-Se bonds was confirmed by several spectroscopic methods. The selenium covalently-modified graphene represents the first example of graphene-selenium compound with potential application in gas sensing. (paper)