Reaction of [Mn 12 (CH 3 COO) 16 (H 2 O) 4 O 12 ]Á2CH 3 COOHÁ4H 2 O and Na 9 [AsW 9 O 33 ]Á19.5H 2 O in aqueous solution leads to the isolation of a new carboxy group decorated TMSP, Na 15 [(Mn II (COOH)) 3 (AsW 9 O 33 ) 2 ]Á15H 2 O (1). The structure of 1 was characterized by elemental analysis, thermometric analysis and single crystal X-ray diffraction (Crystal data for 1: Trigonal, P-3, a = 12.8804(4) Å , b = 12.8804(4) Å , c = 30.3501(19) Å , V = 4360.6(3) Å 3 , Z = 2). Structural analysis reveals that 1 represents the first example of carboxy group decorated arsenotungstate. Antitumor study indicates that compound 1 possesses high antitumor activity against Hela cells. In addition, the magnetic properties of 1 were also investigated.Electronic supplementary material The online version of this article (