Isoprene-emitting plants lose a large portion of their assimilated C as isoprene. Because isoprene synthesis can be regulated, it has been assumed that isoprene benefits the plant. Since the rate of isoprene emission from leaves is highly responsive to temperature, we hypothesized that isoprene benefits plants by increasing their thermotolerance. We used three methods to measure isopreneinduced thermotolerance in leaves. Each technique assayed thermotolerance under conditions that suppressed endogenous isoprene synthesis. When measured by chlorophyll fluorescence, thermotolerance of kudzu (Pueraria lobata [Willd.] Ohwi.) leaves increased as much as 4[deg]C in very low light. With higher light, isoprene increased thermotolerance of kudzu leaves by as much as 10[deg]C. When measured as the temperature at which photosynthesis declined to zero, thermotolerance increased with added isoprene by 2.5[deg]C. All three measures of thermotolerance were dose dependent. Both fluorescence techniques also showed isoprene-induced thermotolerance in white oak (Quercus alba L.). Thermotolerance was not observed in bean (Phaseolus vulgaris var Linden), a species that does not emit isoprene. None of the experiments was designed to determine the mechanism of thermotolerance, but we theorize that isoprene functions by enhancing hydrophobic interactions in membranes.