We present high frequency Electron Spin Resonance (ESR) results on a peapod sample. In a previous low frequency ESR study on peapods and double-walled nanotubes the observation of a non-Lorentzian line shape was reported. This effect was tentatively attributed to the presence of two distinct Lorentzian signals, coming from the outer tube and from the inner tube or from fullerenes. Here we report ESR measurements in a broad frequency (and magnetic field) range, which help to resolve this uncertainty. Since, with increasing magnetic field, no splitting of the line is observed, just a linear increase with frequency, we attribute the non-Lorentzian shape to g-factor distribution. The extracted line width weakly increases with temperature. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim