The speciation in the phosphitomolybdate system, H+-MoO4(2-)-(HP)O(3)2-, has been determined from combined potentiometric and 31P NMR measurements in 0.600 M Na(Cl) medium at 298(1) K. Potentiometric titration data were collected in the ranges 2.5<-log[H+]<6.2, 40.0<or=[Mo]tot<or=240.0 mM, 10.0<or=[P]tot<or=40.0 mM and 1<or=[Mo]tot/[P]tot<or=10. 31P NMR data were collected in the same ranges with the exception that -log[H+] was decreased to 0.8. The pKa values of phosphite were determined by means of potentiometric (1.5<-log[H+]<8.6) and 31P NMR chemical shift data (0.0<-log[H+]<12.0). The following log beta (3sigma) values were deduced: H(HP)O3-, 6.09(2); H2(HP)O3, 7.23(4) (pKa=1.14). In the phosphitomolybdate system, the following complexes were found along with their formation constants ((HP) denotes phosphite): Mo5(HP)2(4-), 65.31(4); Mo6(HP)(2)5-, 71.94(5); Mo5(HP)4-, 51.25(8); Mo5(HP)3-, 54.9(3); Mo6(HP)2-, 68.99(9); Mo7(HP)5-, 71.25(3); Mo7(HP)4-, 74.2(2). Because hydrolysis and oxidation of phosphite to phosphate were observed by NMR, with a mixed-hetero species of low [Mo]tot/[P]tot ratio being detected, the four-component H+-MoO(4)2--(HP)O3(2-)-HPO(4)2- system was also studied in the same way over the range 0.8<-log[H+]<6.2 and [Mo]tot/[P]tot fixed at 1.5. The following mixed heteropolyanion species have been found (P denotes the phosphate group): Mo5(HP)P5-, 62.27(6); Mo5(HP)P4-, 66.76(4). The equilibrium speciation in the two systems are illustrated by distribution diagrams. In addition, the aqueous structures are proposed for all the species formed in the systems.