Two new dinuclear sandwich-type heteropolymolybdates based on the mulitidendate inorganic fragment [AsMo7O27] and Cr(III) and Fe(III) ions, namely, the homometallic sandwich polyoxometalate (POM) (NH4)(12)[Fe-2(AsMo7O27)(2)]center dot 12H(2)O (1) and the first example of the "symmetrical" heterometallic Cr(III)-Fe(III) sandwich POM, (NH4)(12)-[FeCr(AsMo7O27)(2)] center dot 13H(2)O (2), were simultaneously synthesized in high yield. Their magnetic properties are thoroughly investigated together with the homometallic sandwich POM (NH4)(12)[Cr-2(AsMo7O27)(2)]center dot 11H(2)O (3). The Xm(T) values for compounds 1-3 at 300 K correspond well to the calculated spin-only values for Fe-III (S=5/2) and Cr-III S=3/2) With g(Fe) = g(Cr) = 2. Upon cooling, the Xm(T) values decline monotonously and reach 0.14, 1.00, and 0.11 cm(3) K mol(-1) at 2.0 K for 1, 2, and 3, respectively, indicating a significant antiferromagnetic exchange between the magnetic centers with J = -2.09, -4.09, and -6.26 cm(-1), respectively, for 1, 2, and 3. The magnetic results clearly establish that compound 2 is formed by bimetallic Cr(III)-Fe(III) units and not by a mixture of the two antiferromagnetically coupled homometallic species. Their thermal properties are also characterized.