Reaction of 2,4-(naphthalene-1,8-diyl)-1,3,2,4-dithiadiphosphetane 2,4-disulfide 1 with hexamethyldisilazane (hmds) in acetonitrile gave the N,N'-bis(trimethylsilyl)acetamidinium salt of the [(C10H6)P(S)(NHSiMe3)SP(S)(2)](-) anion 2, a product of P,S, ring cleavage, together with the new thiazaphosphetidine (C10H6)P(S)SN(SiMe3)P(S) 3. Analogous (methoxy derived) products to 2 (2a, 2b) and 3 (3a) were obtained when 2,4-(4-methoxynaphthalene-1,8-diyl)-1,3,2,4-dithiadiphosphetane 2,4-disulfide 1a was used instead of 1. When the reaction of 1 with hmds was performed in dichloromethane a mixture of the products was obtained, from which the hexamethyldisilazan-2-ium salt of the anion of identical structure to that in 2 has been isolated. No reaction occurred when a solventless system was used. The new compounds were studied spectroscopically (one-and two-dimensional NMR, IR spectroscopy) and by X-ray crystallography (3, 3a and 4).