Elevated concentrations of atmospheric CO2 are likely to interact with other factors affecting plant physiology to alter plant chemical profiles and plant–herbivore interactions. We evaluated the independent and interactive effects of enriched CO2 and artificial defoliation on foliar chemistry of quaking aspen (Populus tremuloides) and sugar maple (Acer saccharum), and the consequences of such changes for short-term performance of the gypsy moth (Lymantria dispar). We grew aspen and maple seedlings in ambient (˜360 ppm) and enriched (650 ppm) CO2 environments at the University of Wisconsin Biotron. Seven weeks after budbreak, trees in half of the rooms were subjected to 50% defoliation. Afterwards, foliage was collected for chemical analyses, and feeding trials were conducted with fourth-stadium gypsy moths. Enriched CO2 altered foliar levels of water, nitrogen, carbohydrates, and phenolics, and responses generally differed between the two tree species. Defoliation induced chemical changes only in aspen. We found no significant interactions between CO2 and defoliation for levels of carbon-based defenses (phenolic glycosides and tannins). CO2 treatment altered the performance of larvae fed aspen, but not maple, whereas defoliation had little effect on performance of insects. In general, results from this experimental system do not support the hypothesis that induction of carbon-based chemical defenses, and attendant effects on insects, will be stronger in a CO2-enriched world.