Flux perturbations due to indium disc foils exposed to an isotropic thermal neutron flux in a water medium were experimentally determined. The unperturbed flux, or the “zero thickness” activity, was determined using foils which matched the moderating ratio of the medium, thereby causing no flux perturbation. Comparison of the results with several modifications of the Ritchie and Eldridge theory gives excellent agreement for foil thickness from 0 to 0.025 in.