There is increasing public awareness and concern over emissions from municipal solid-waste incinerators. Two well-recognised factors are the value of knowing the gas-residence times in large municipal incinerators, and the serious error imposed by the traditional use of average gas-residence time based on volume flow rate. The problem of measuring gas-residence time in large incinerators was studied by the pseudo-random binary sequence (PRBS) stimulus tracer response technique at the Sheffield municipal solid-waste incinerator (35 MW plant). The steady-state system was disturbed by the superimposition of small fluctuations in the form of a pseudo-random binary sequence of methane pulses, and the response of the incinerator was determined from the CO2 concentration in flue gases at the boiler exit, measured with a specially developed optical gas analyser with a high-frequency response. For data acquisition, an on-line PC computer was used together with the LAB Windows software system; the output response was then cross-correlated with the perturbation signal to give the impulse response of the incinerator. There was Very good agreement between the gas-residence time for the Sheffield MSW incinerator as calculated by computational fluid dynamics (FLUENT Model) and gas-residence time at the plant as measured by the PRBS tracer technique. The results obtained from this research programme clearly demonstrate that the PRBS stimulus tracer response technique can be successfully and economically used to measure gas-residence times in large incinerator plants. It also suggests that the common commercial practice of characterising the incinerator operation by a single-residence-time parameter may lead to a misrepresentation of the complexities involved in describing the operation of the incineration system. The proposed legislative criterion of a 2-seconds gas-residence time at 850 degrees C is questionable in relation to the complete combustion of waste and the formation of toxic gaseous products in municipal incinerators. In particular there is a need for accommodation of other factors, such as the interaction between chemical kinetics and turbulent mixing, the potential role of non gas-phase effects on the formation of PCCDs (dioxins), and the effect of the physical form of the waste on the completeness of incineration.