The Dumas method is a protocol with methodology allowing the molecular weight of an unknown substance to be determined. The molecular weight of a compound is the sum of the atomic weights of the atoms which comprise the molecule. For instance, the molecular weight of water (H₂O) is equal to the atomic weight of oxygen plus the atomic weights of two hydrogen atoms. Often, the Dumas method is used to determine the molecular weights of volatile organic substances that are liquids at room temperature.

Using the Dumas method, molecular weight is calculated by measuring the mass of a known volume of a vaporized liquid. The mass of the vapor produced is measured by condensing it into liquid. The Ideal Gas Law, PV=nRT, is then used to determine the volume of the gas, and from that, its density. In the Ideal Gas Law, P is pressure, V is the volume of gas in liters, n is the number of moles of the gas, R is the Ideal Gas Constant, and T is the temperature of the gas (on Kelvin scale). From the density of the substance, the molecular weight can be estimated. Once the molecular weight of a substance is known, estimates of its atomic composition, or which elements it is made from and in what proportions, can be made. The method was designed by a french chemist named Jean Baptiste André Dumas, after whom the procedure is now named. Even though, during the nineteenth century the science of chemistry was at a relatively early state, Dumas was able to show that the vapor densities of some organic compounds are directly proportional to their molecular weights.