The maximum losses occurring at very short wavelengths, due to demagnetization in γFe2O3tape, have been calculated to be rather small (4.5 dB and 8 dB for magnetic and non-magnetic head reproduction, respectively) and they are therefore difficult to measure in the face of the relatively large and indeterminate spacing loss. A method is described for testing a particular tape in free space which avoids the difficulty. Essentially, a determination of the incremental remanent susceptibility of the tape is made. Because the remanence curve is so highly nonlinear, this susceptibility is uniquely associated with the demagnetizing field within the tape. Whereas the demagnetizing fields thus found agree with theory for wavelengths greater than 1 mil, they decrease, contrary to expectations, at the shorter wavelengths.