Magnetic exchange interactions in the heteropoly complexes [M4(H2O)2(PW9O3 4)2]10− [M=Co(II) and Cu(II)] J.Coupled pairs of differing magnetic spins in tungsto-heteropoly complexes have been investigated by studying the magnetic susceptibility over the temperature range 2-300 o K. The two sites of the magnetic ions are surrounded by oxygen ions in octahedral and tetrahedral coordinations with one oxygen ion in common. The ions studied were CoH, CoH, and Fe H in a total of six combinations on the two sites, offering a unique set of symmetries and ions for examining super-exchange effects. A closed form for the susceptibility has been obtained on assuming the spin Hamiltonian 3C ={J(gISI+g2S.) ·H-JSI· S2, allowing for the possibility of gl,e g2, Sl,e S2. Fitted values of J /k range from -6 to -70oK depending on the combinations of ions. The experimental agreement with the spin Hamiltonian for all cases studied is satisfactory for this system.