When flux measurements are made in reactors or in piles, large ion chambers are commonly used. The current output of these chambers is read in terms of flux. The chambers depress the flux, however, and a correction should be applied to get the value of the unperturbed flux. The flux perturbation was measured in a large graphite diffusing medium, the Argonne National Laboratory Standard Pile, and found to be between 5% and 25% when measured on the outer surface of typical ion chambers. At about 10 in. from the end of the chamber the perturbation was no longer observed. The flux was measured with a small fission counter which, of itself, did not depress the flux appreciably. To measure the flux depression inside an ion chamber, the latter was simulated by stacking boron-coated aluminum plates above and below the small fission counter used previously. The measurement of the flux depression was found to be in good agreement with that which can be estimated from a calculation in which an exponential absorption is assumed. From these experiments it is concluded that the value of the flux measured with a large boron coated ion chamber gives an estimation of the flux within 20% to 50% of the unperturbed value depending on the amount of boron in the chamber, while the estimation of the flux is within 5% to 15% when measured with a large U235-coated fission counter. It should be noted that, although these results apply in a graphite diffusing medium, they do not necessarily apply in an absorbing medium such as the heavy concrete which usually surrounds the instrument holes in reactors.