ABSTRACT. We include short and elementary proofs of two theorems that characterize reductive group schemes over a discrete valuation ring, in a slightly more general context. MSC 2000: Primary 11G10, 11G18, 14F30, 14G35, 14G40, 14K10, and 14J10. \ KEY WORDS: Group schemes and discrete valuation rings. §1. IntroductionLet k be a field. Let p ∈ {0} ∪ {n ∈ N|n is a prime} be the characteristic of k. Let V be a discrete valuation ring of residue field k. Let π be a uniformizer of V and let] be the field of fractions of V . Let F = Spec(P ) and G = Spec(R) be flat, affine group schemes over V . We will assume that F is a reductive group scheme over V ; so F is smooth over V and its fibres are connected and reductive groups over fields. In this paper we present elementary and short proofs of the following two basic theorems on reductive group schemes over V . (b) If p = 2, we assume that the group GK has no normal subgroup that is isomorphic to SO 2n+1 for some n ∈ N. Then G is a reductive group scheme over V .(c) The normalization G n of G is a finite G-scheme. Moreover, there is a faithfully flat V -algebraṼ that is a discrete valuation ring and such that the normalization (GṼ ) n of GṼ is a reductive group scheme overṼ .