The equilibrium speciation in the pH++qMoO42–+rH2O2+sH2PO4–+tSO42–(H+)p(MoO42–)q(H2O2)r(H2PO4–)s(SO42–)tp–2q–s–2t system in 0.300 M Na2(SO4) medium at 25 °C and [H2O2]tot/[Mo]tot > 2 has been determined from potentiometric data and 31P NMR integral and chemical shift data in the range 1.0 pH 9.0, 2.0 [MoO42–]tot/mM 293, 32 [H2O2]tot/mM 640, 2 [H2PO4–]tot/mM 60 and 200 [SO42–]tot/mM 380. Species with the following compositions were obtained: MoX2P3–(0,1,2,1,0), MoX2P2–(1,1,2,1,0), MoX2P–(2,1,2,1,0), Mo2X4P3–(2,2,4,1,0), Mo2X4P2–(3,2,4,1,0), Mo3X6P3–(4,3,6,1,0), Mo3X6P2–(5,3,6,1,0) and Mo4X8P3–(6,4,8,1,0). The numbers and charges of molybdenum (Mo), peroxide (X), phosphate (P) and sulfate (S) in each species are given in the abbreviated formula MoqXrPsStp–2q–s–2t. The numbers in parentheses refer to the values of p, q, r, s and t in the formula above. The following formation constants with 3 were obtained; log0,1,2,1,0= 5.16 ± 0.09, log1,1,2,1,0= 12.73 ± 0.02 (pKa= 7.63), log2,1,2,1,0= 16.14 ± 0.03 (pKa= 3.42), log2,2,4,1,0= 25.03 (± 0.04), log3,2,4,1,0= 29.54 ± 0.02 (pKa= 4.51), log4,3,6,1,0= 42.30 ± 0.03, log5,3,6,1,0= 44.06 ± 0.08 (pKa= 1.76), log6,4,8,1,0= 57.30 ± 0.07. pKa values for H3PO4 and H2PO4– were determined to 2.00 and 6.48, respectively. Chemical exchange processes were detected by 31P NMR 2D EXSY and selective magnetisation transfer experiments between the complexes. An averaged lifetime, (X2Mo–OP) 30 ms, can be estimated for the MoX2P and Mo2X4P species at pH = 5.5 and 5 °C.