An apparatus and method for determining the average molecular weight of a complex mixture from a plurality of test samples of the complex mixture are disclosed. The method includes a) providing a chamber having two fixed conditions and one variable condition selected from three state variables of temperature, pressure and volume; b) introducing a test sample of known mass into the chamber; c) setting operating conditions of the state variables such that the test sample will be fully vaporized into a gaseous state without decomposition or chemical reaction; and d) measuring the change of the variable condition after full vaporization of the test sample is achieved. Steps b-d are repeated for several test samples having different masses. The average molecular weight of the complex mixture is determined as a linear gradient (slope) of the relationship for the change of variable condition as a function of test sample mass.