Two new isostructural minerals udinaite and arsenudinaite with the end-member formulae and, respectively, are found in the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. They ane associated with one another and anhydrite, diopside, hematit2, schăferite, berzeliite, svabite, calciojohillerite, tilasite, reznitskyite, ludwigite, rhabdoborite-group borates, forsterite, magnesioferrite, fluorapatite, pliniusite, and powellite. Both minerals cocur as equant thetragonal prismatic-dipy ramidal crystals up to, aggregates up to and interrupted crusts up to . Udinaite and arsenudinaite, visually indistinguishable from one anothes, are transparent, beige or brownish-yellowish, with viteous lustre. Both minerals are optically uniaxial ; and (udinaite/ arsenudinaite). The empiriminerals are tetragonal, and (udinaite/arsenudinaite). Their crystal structure consists of the helical chains of edge-sharing octahedra and isolated tetrahedra, forming a heteropolyhedral pseudo-framework with Na cations located in cavities. Both minerals are isostructural to jeffbenite. Udinaite and arsenudinaite form an isomorphous series in which the contents of constituents vary within (in apfic): V_(16-1) As_(2s-10) P₀₄₀₀.