Quantum corrections of Abelian duality transformations

A modification of the Abelian duality transformations is proposed guaranteeing that a (not necessarily conformally invariant) sigma-model be quantum equivalent (at least up to two loops in perturbation theory) to its dual. This requires a somewhat nonstandard perturbative treatment of the dual sigma-model, Explicit formulae of the modified duality transformation are presented for a special class of block diagonal purely metric sigma-models.