A radiation curable composition is disclosed that includes a curable cross-linker essentially free of urethane and urea functional groups, a curable diluent, and a non-radiation curable component comprising (thio)urethane and/or urea groups. Coated optical fibers having a primary coating formed from this radiation curable composition, as well as optical fiber ribbons that contain the coated optical fibers are disclosed. Methods of making the optical fibers and ribbons are also disclosed.