  We use chiral perturbation theory to evaluate the scattering amplitude for
the process Pi^+ K^- to Pi^+ K^- at leading and next-to-leading orders in the
chiral counting and in the presence of isospin breaking effects. We also
discuss the influence of the latter on the combination of the S-wave Pi K
scattering lengths which is relevant for the 2S - 2P energy levels shift of K
Pi atoms.
