The breaking of internal waves on a sloping surface in a two-fluid system is studied experimentally in a wave tank. The characteristics of breaking internal waves are examined through their frequency spectra for cases of different frequency, amplitude, and wavelength. Waveheight measurements are made with capacitance probes in conjunction with a recently developed capacitance bridge and are processed by computer utilizing the Cooley-Tukey fast Fourier transform algorithm. By using a direct, on-line computer system spectra can be computed with a high degree of accuracy and a very narrow bandwidth. Spectra measured in the breaking region of the tank reveal a series of sharp peaks in the low frequency range representing the different harmonics of the internal waves. These higher modes, which possess little or no energy in the incident wave, gain energy at the expense of the lowest mode as the wave train propagates over the sloping surface. The result is a transfer of energy to higher frequencies through growth of the harmonics, each mode gaining energy at the expense of lower modes and transferring it to higher modes. At higher frequencies the spectra no longer possess these distinct harmonics. The high frequency end of the spectrum in the breaking region has a slope of approximately −3 for all cases considered.