The analysis of modulation-broadened experimental magnetic resonance spectra by the fitting of theoretical lineshape functions is described together with tests on zero field NMR lineshapes from ⁵⁹Co in cubic cobalt. It is shown that for small modulation amplitudes values for the resonance width, intensity and mode admixture may be obtained. For large modulation amplitudes considerable errors may occur in the deduced values of intensity and width but their ratio, the resonance amplitude, is still accurately determinable.