In this paper we present a simple yet novel technique for harmonic/percussive separation of monaural audio music signals. Under the assumption that percussive/harmonic components exhibit vertical/horizontal lines in the spectrogram, image morphological filters are applied to the spectrogram of the input signal. The structure elements of the morphological filters are chosen to accentuate regions of the spectrogram corresponding to harmonic and percussive components. The proposed method was evaluated on the SISEC 2008/2010 development data and outperformed the baseline method adopted.