This paper presents investigations into the use of ionic liquids as sizing agents for carbon fibers in epoxy matrices. A variety of aqueous ionic liquid solutions was employed as the sizing bath, at concentrations relevant to industry standards. Of the ionic liquids used, the widely available and inexpensive BmimCl was found to give the optimal fiber-to-matrix adhesion improvements of 250% relative to control fibers in epoxy, and 66% improvement in polypropylene. Molecular dynamics simulations suggest that the diffusion of the ionic liquids into the polymer phase accompanied by self-agglomeration generates a plasticised interphase resulting in high shear tolerant materials.