The present invention relates to sizing compositions, to glass fibers at least partially coated with such sizing compositions, to a variety of fiber glass products at least partially coated with such sizing compositions, and to composites formed from cycloolefins and reinforced with glass fibers at least partially coated with sizing compositions of the present invention. In one non-limiting embodiment, a sizing composition for glass fibers comprises a film-former comprising a polymer, wherein the repeating unit of the polymer comprises at least four carbon atoms and at least one carbon-carbon double bond, and a silane. A sizing composition for glass fibers, in another non-limiting embodiment, comprises a film-former comprising a polymer, wherein the repeating unit of the polymer comprises four to six carbon atoms and one carbon-carbon double bond, in an amount between about 60 and about 90 weight percent on a total solids basis, and a silane comprising a terminal vinyl group in an amount between about 5 and about 25 weight percent on a total solids basis.