Purpose. To determine whether aging vitreous collagen fibrils undergo ultrastructural changes that might underlie vitreous liquefaction and posterior vitreous detachment. methods. Vitreous collagen fibrils from 21 human subjects (age range, 3–89 years) and from bovine eyes were isolated on electron microscopy grids. Cupromeronic blue labeling in the presence of 0.3 M MgCl2 and immunogold labeling for collagen types II and IX were analyzed by transmission electron microscopy. results. Aging was associated with marked changes on the surface of human vitreous collagen fibrils, including an exponential loss of type IX collagen along with its chondroitin sulfate side-chains (half-life, 11 years) and a fourfold increase in the exposure of type II collagen. conclusions. Despite being a minor component of vitreous collagen fibrils, type IX collagen, probably by virtue of its chondroitin sulfate side-chains, shields type II collagen from exposure on the fibril surface. With aging, this shielding diminishes, resulting in the surface exposure of “sticky” type II collagen and thus predisposing the vitreous collagen fibrils to fusion. These changes could underlie vitreous liquefaction and weakening of vitreoretinal adhesion.