Abstract. Given a reductive group G over a base scheme S, Brylinski and Deligne studied the central extensions of a reductive group G by K 2 , viewing both as sheaves of groups on the big Zariski site over S. Their work classified these extensions by three invariants, for S the spectrum of a field. We expand upon their work to study "integral models" of such central extensions, obtaining similar results for S the spectrum of a sufficiently nice ring, e.g., a DVR with finite residue field or a DVR containing a field. Milder results are obtained for S the spectrum of a Dedekind domain, often conditional on Gersten's conjecture.