In this work, 1 wt % carbon monoxide (CO) poly(ethylene-carbon monoxide) (ECO) copolymer sheets were artificially exposed to ultraviolet (UV) light with a power density of 3 mW/cm(2) for up to 130 h. A thorough mechanical characterization of the irradiated material was conducted, in which both the stress-strain data and the values of the quasistatic crack initiation and growth toughness were measured and correlated with companion uniaxial tensile tests and single-edge-notched fracture tests. Average values of the elastic modulus, failure strain, and failure stress were determined from the tensile tests. The full-field optical technique of digital image correlation was used to quantify in-plane deformation (displacements and displacement gradients) during the fracture experiments and to extract values of the crack initiation and growth fracture toughness. The elastic modulus increased monotonically with UV irradiation for the exposure times used in this investigation. In addition, for low irradiation times of less than 5 h, both the failure strain and failure stress of ECO decreased, and this caused a corresponding decrease in the crack initiation and growth toughness. However, for longer irradiation times, the failure strain remained almost invariable, whereas the failure stress increased by about 25% over that of unirradiated ECO. As a result, for longer irradiation times (>5 h), 1 wt % CO ECO became not only stiffer but also stronger and tougher, as quantified by companion fracture experiments. (C) 2004 Wiley Periodicals, Inc.