  On the basis of a next-to-leading-order calculation in chiral perturbation
theory, the first complete analysis of isospin breaking for direct CP violation
in K^0 -> 2 pi decays is performed. We find a destructive interference between
three different sources of isospin violation in the CP violation parameter
epsilon'. Within the uncertainties of large-N_c estimates for the low-energy
constants, the isospin violating correction for epsilon' is below 15 %.
