Optical fibers used to transport sunlight exhibit considerable light leakage within their nominal numerical aperture. Of particular interest in the design and diagnosis of solar fiber-optic concentrators is the dependence of this leakage on: (a) incidence angle, (b) the optical properties of the core and the cladding. and (c) fiber length. We present measurements of fiber angular transmission. along with a theoretical model. The implications for solar fiber-optic concentrators are also assessed. (C) 2002 Elsevier Science Ltd. All rights reserved.