A method of making cellulose ester polymer comprises the steps of: pre-treating a cellulose source; esterifying the pretreated cellulose source to form a cellulose ester; and optionally hydrolyzing the cellulose ester. The pretreatment step further comprises: mixing, in a first stage, the cellulose source with a first solution of a weak acid and water, whereby the cellulose source being suspended in the first solution; de-liquefying the first stage suspension; mixing, in a second stage, the de-liquefied mass with a second solution of weak acid and water, whereby the mass being suspended in the second solution; and de-liquefying the second stage suspension.