A connection is shown between the rise in electron temperature and the presence of plasma instabilities during an ionospheric heating experiment. The heating of the electron gas is due to the combined effects of both ohmic and instability energy deposition. The instabilities are generated by the intense electric fields near the altitude of reflection of the O mode HF wave. The dominant instability is the decay of the HF wave into a Langmuir wave and on ion acoustic wave. These daughter waves then rapidly lose energy to the local electrons. When the HF transmitter is cycled on and off to measure the rate of change of electron temperature, the plasma instability intensity has an overshoot behavior. This change in instability level in time permits a distinction between the ohmic and the instability energy sources.