A P0± calculation indicates that neutron detecting foils will cause no flux perturbation when prepared with a moderating ratio equal to that of the medium in which they are to be used. Such foils can be easily prepared from an appropriate solvent and solute. The activity in these foils can be conveniently determined on an absolute basis using liquid scintillation counting techniques, thus yielding an accurate value of absolute neutron flux. Measurements in polyethvlene are reported.