The water vapour nonequilibrium condensation phenomenon existing in combustion—heated wind tunnels can affect nozzle flow characteristics. An experimental system was designed and built to simulate this process at submillisecond time scales. An absorption spectroscopy (TDLAS) measurement technique was used to obtain information on the changes of the flow field temperature and the water vapour concentration. The experimental results show that water vapour condensation has a similar pattern on a submillisecond time scale: the temperature shows a trend of first decreasing, then increasing and then decreasing, while the water vapour concentration starts to decrease after a period of expansion. When the length of the test section is the only variable, the shorter the length of the test section is, the shorter the condensation time and the lower the temperature of the inflection point. This means that in nozzles with different sizes, smaller nozzles will have a shorter condensation time and higher inflection point temperature.