A procedure is outlined for calculating the maximum changes in the exhaust velocity of gases which can be introduced during flow through a rocket nozzle as the result of vibrational time lags. Representative calculations have been carried out for a number of propellant systems. These calculations indicate that the vibrational time lags cannot produce changes in exhaust velocity which exceed a few percent. The numerical values of the changes are found to depend not only on the nature of the gases but also on the occurrence or non‐occurrence of chemical reaction during flow.