Gas-sampling systems have shown an inability to respond to the rapid changes in conditions normally experienced in solid-waste incinerators, and this has led to control problems. Even in large municipal incinerators, oxygen deviations take place in timescales comparable to the gas-sampling lag. The spectroscopy-based system-the peak-seeker-described here has the potential to solve these problems. The research, design and development of a computerised peak-seeker control system that may be applied to the secondary tone of a waste incinerator is also described. The system relies on the measurement of flame radiation in response to a sinusoidal air/fuel ratio modulation. Tests made on a laboratory-based oil furnace include multisine system characterisation and the effect of fuel flow on the Control point. Novel developments to the peak-seeker have been made which allow more information to be gained on the operating point. The incorporation of the peak-seeker into a commercial incinerator unit is also described, making use of a fluidic valve (no moving parts) to improve life.