Study of supersonic flow over wall-mounted cavities for two different length/depth (L/D) ratios is carried out experimentally. Unsteady pressure measurements were made on the front and aft walls of the cavity. Data analysis was performed on the experimental results so obtained. Spectra of the unsteady pressure data exhibit multiple tones. Higher-order spectral technique is implemented on the unsteady pressure data to ascertain whether these multiple tones are due to possible nonlinear interactions between the primary cavity modes (Rossiter modes) or not. Significant nonlinear interactions in the form of both sum and difference frequencies between the cavity modes are observed in both the cavities. The spectra of the cavity with L/D ratio 2 show distinct peaks due to nonlinear interactions while the cavity with L/D ratio 3 does not exhibit observable peaks in the spectra. The spectra of both the cavities show presence of low-frequency peaks of significant amplitudes. These low-frequency modes interact with the primary cavity modes to produce significant bicoherence values. The reasons for their existence could not be predicted. It is identified that the dominant mode in the spectra of the cavities is critical for most of the interactions observed. © Springer-Verlag 2012.