Abstract. Let (G, I, N, S) be an affine topological Tits system, and let Γ be a torsion free cocompact lattice in G. This article studies the coinvariants H 0 (Γ; C(Ω, Z)), where Ω is the Furstenberg boundary of G. It is shown that the class [1] of the identity function in H 0 (Γ; C(Ω, Z)) has finite order, with explicit bounds for the order.A similar statement applies to the K 0 group of the boundary crossed product C * -algebra C(Ω) ⋊ Γ. If the Tits system has type A 2 , exact computations are given, both for the crossed product algebra and for the reduced group C * -algebra.