The present invention relates to a reinforced composite material and a method for its production. The composite material comprises at least one cured resin having a reinforcing material. Preferably the reinforcing material is a plurality of glass fibers which are treated such that the properties of the interphase substantially surrounding each fiber are substantially equivalent to those of the bulk cured resin. The fiber treatment may be selected from the group consisting of a polymeric coating, a hydrophilic surface coating, a surface coating of a free radical inhibitor, or a reduction in the total surface area of the fibers. The reinforced composite material of the invention provides improved long-term mechanical properties compared to traditional glass fiber reinforced materials.