View of the intermolecular interactions N–H⋯O and O–H⋯O forming polymeric dihydrogendecavanadate(V)–decavanadate(V) chains running along the 1 1 ¯ 1 axis in NH 2(CH 2) 4 5V 10O 28H 2 0.5V 10O 28 0.5. A new polymeric compound, NH 2(CH 2) 4 5V 10O 28H 2 0.5V 10O 28 0.5, was obtained by in situ synthesis of the organic cation from an aqueous solution of V 2O 5–HCl–NH 2(CH 2) 4NH 2. The crystal structure was solved by single-crystal X-ray diffraction. NH 2(CH 2) 4 5V 10O 28H 2 0.5V 10O 28 0.5 consists of dihydrogendecavanadate(V), H 2V 10O 28 4−, and decavanadate(V), V 10O 28 6−, units assembled in one-dimensional arrays by interanionic hydrogen bonds O–H⋯O and cation-anion N–H⋯O interactions. The latter involve pyrrolidinium cations, which were obtained in the compound instead of the starting butane-1,4-diamine. Pyrrolidinium cations further connect the polymeric chains into a three-dimensional network. The presence of the two types of units, H n V 10O 28 (6− n )− with n = 0 and n = 2, in one compound was not yet observed and is herein reported for the first time.