Abstract. In this paper we prove that the moduli space of rank 2n symplectic instanton bundles on P 2n+1 , defined from the well-known monad condition, is affine. This result was not known even in the case n = 1, where by Atiyah, Drinfeld, Hitchin, and Manin in 1978 the real instanton bundles correspond to self-dual Yang Mills Sp(1)-connections over the 4-dimensional sphere. The result is proved as a consequence of the existence of an invariant of the multidimensional matrices representing the instanton bundles.