A “thermal pulse” technique has been used to measure thermal diffusivity and thickness of combustion chamber deposits continuously during engine operation. The technique uses a fast-response thermocouple junction at the combustion chamber wall surface and a simplified model which describes the effect of the deposit on the measured temperature cycle. Results from 13 tests using four different fuels and three different commercial additive packages are discussed in the paper. Thermal diffusivity values in the range of 0.85 - 4.2 x 10-7 m2s-1 were measured. Deposit growth is normally a continuous process. However, occasionally deposit flaking events characterised by a sudden significant decrease in deposit thickness were observed.