A low residence time flow method for the measurement of the critical temperatures and pressures of thermally-unstable fluids was developed in this work. The low residence time at elevated temperatures minimizes decomposition and other reactions, making it possible to measure the critical properties of many unstable fluids. An extrapolation procedure was developed to obtain the critical properties of rapidly reacting substances, based on the linear behavior of the apparent critical property with residence time. The measured critical properties of 14 n-alkanes (pentane through octadecane) using this method are reported in this paper. The results extend the available data on the critical properties of the higher alkanes and reveal a possible error in the literature value for the critical pressure of tetradecane.