We report the first experimental determination of the microscopic strain fields surrounding substitutional impurities in a crystal. We have used pair spectra in GaP to reveal the splitting of Zn or C acceptor levels in the strain fields generated by O, S, Se, or Te donor ions. The anisotropic splitting, which agrees with a simple effective-mass theory, is the same for all four donors. We conclude that effects associated with the size of these impurities are distributed over a volume of the crystal extending more than 20 from the impurity. © 1971 The American Physical Society.