Abstract: This invention provides a method for controlling the reaction between cellulose and an etherifying agent which comprises the steps of (a) combining cellulose with an alkylene oxide and alkyl halide in a reactor wherein an etherification reaction is initiated which includes a vapor phase, (b) periodically measuring the quantity of at least one of the reactants or products in the vapor phase of the reactor and therewith determining the rate of the etherification reaction taking place, and (c) regulating the reaction temperature continuously while the reaction is taking place by using the rate of reaction determined in step (b) to determine the amount of heating or cooling needed for the reactor.