Abstract: A self-hardening moulding mixture for making foundry moulds or cores, comprising moulding sand, orthophosphoric acid, a ferrous oxide-containing material, a water-soluble salt of an alkali metal or ammonium, and a carboxylic acid having one dissociation constant not exceeding 10.sup.-4 or a water-soluble salt of an alkali metal or ammonium, and an isopolyacid formed by a chromium subgroup metal, manganese oxide or urea.The self-hardening moulding mixture can also comprise a surface-active substance acting as a foaming agent in order to render the mixture to a fluid state and to avoid compacting when producing moulds and cores therefrom.The moulding mixture features an enhanced bench life (10-15 min) and ensures a sufficiently high hardening and strength rates (compression strength is equal to 10-25 kg/cm.sup.2) for the foundry moulds and cores produced therefrom.