The magnetic susceptibility and low-temperature magnetization curve of the [3 x 3] grid [Mn(III)(4)Mn(II)5(2poap-2H)(6)](CIO4)(10)(.) 10H(2)O (1) are analyzed within a spin Hamiltonian approach. The Hilbert space is huge (4 860 000 states), but the consequent use of all symmetries and a two-step fitting procedure nevertheless allows the best-fit determination of the magnetic exchange parameters in this system from complete quantum mechanical calculations. The cluster exhibits a total spin S = 1/2 ground state; the implications are discussed.