The cyclotriphosphate hexahydrate of manganese and silver, MnAg4(P3O9)(2).6H(2)O, was prepared by Boulles process. MnAg4(P3O9)(2).6H(2)O crystallizes in the triclinic system, P-1, Z = 1 and its structure was determined by Rietveld's method. The refinement of 54 parameters by this method, using 1752 independent reflections leads to R-wp = 0.098, R-p = 0.065 and R-B = 0.033 on the basis of its isotype NiAg4(P3O9)(2).6H(2)O. The unit cell parameters of this salt are the following a = 8.824(1) Angstrom, b = 8.485(1) Angstrom, c = 6,609(1) Angstrom, alpha = 90.30(1)degrees, beta = 92.89(1)degrees and gamma = 107,28(1)degrees. The thermal behaviour of this new compound was studied, between 25 and 600 degreesC, under atmospheric pressure by infrared spectrometry, X-ray diffraction, thermal analyses TGA and DTA coupled. This study allows us the identification and the crystallographic characterization of a new phase, MnAg2(PO3)(4) isotype of ZnRb2(PO3)(4), obtained between 350 and 450 degreesC, mixed with the long-chain polyphosphate of silver AgPO3. The kinetic characteristics of the dehydration of MnAg4(P3O9)(2).6H(2)O and the thermal phenomena accompanying this dehydration were determined and discussed on the basis of the proposed crystalline structure.