The free acid, H[Re(NO)(C2O4)(OH)2(H2O)] has been prepared from the reaction of Re(NO)(OH)3 H2O with oxalic acid in aqueous medium. The K+, NH4+ and Pb2+ salts of the acid have been isolated. Nonelectrolytic diimino derivatives, Re(NO)(C2O4)X · L (X = 1, 10‐phenanthroline, 2,2′‐dipyridyl; L OH−, Cl−) have been synthesized. The complexes have been characterised through elemental analyses, spectral (u.v., vis., i.r.) properties, magnetic and conductance data and their structures are proposed. Copyright © 1983 Verlag GmbH & Co. KGaA, Weinheim