K1 of separative exchange rings and C*-algebras with real rank zero

For any (unital) exchange ring R whose finitely generated projective modules satisfy the separative cancellation property (A + A congruent to A + B congruent to B + B double right arrow A congruent to B), it is shown that all invertible square matrices over R can be diagonalized by elementary row and column operations. Consequently, the natural homomorphism GL(1) (R) --> K-1 (R) is surjective. In combination with a result of Huaxin Lin, it follows that for any separative, unital C*- algebra A with real rank zero, the topological K-1 (A) is naturally isomorphic to the unitary group U(A) modulo the connected component of the identity. This verifies, in the separative case, a conjecture of Shuang Zhang.