We have prepared the Ti7-containing, tetrahedral 36-tungsto-4-arsenate(III) [Ti6(TiO6)(AsW9O33)4](20-) (1) by a simple, one pot procedure. Polyanion 1 contains a novel Ti7-core, comprising a central TiO6 octahedron surrounded by six TiO5 square-pyramids, and capped by four {As(III)W9} trilacunary fragments. The title polyanion is solution-stable, as shown by (183)W NMR and mass spectrometry, and exhibits interesting biological properties.