Abstract: Cellulose microfibrils, in particular for use as a reinforcing filler or structuring agent in composite materials, a method for making same, and composite materials containing said microfibrils, are disclosed. Specifically, said microfibrils are surface-modified cellulose microfibrils with an L/D ratio higher than 20, preferably higher than 100, and an average diameter of 1-50 nm, where L is the length of the microfibril and D is its average diameter. At least 25% by number of the hydroxyl functions on the surface of the microfibrils are esterified by at least one organic compound including at least one function capable of reacting with the hydroxyl groups of the cellulose. As a result, when the organic compound is a compound that includes acetyl groups such as acetic acid, said microfibrils are used as a reinforcing filler in a material comprising cellulose acetate as the polymeric matrix. The filled composite material may be shaped to provide films, moldings, fibers or yarns.