Foliar spray applications of 40% aqueous methanol were made to sunlit leaves of sour orange trees that had been grown continuously in clear‐plastic‐wall open‐top enclosures maintained out‐of‐doors at Phoenix, Arizona, for over 5.5 years in ambient air of approximately 400μmol mol‐1CO2and in air enriched with CO2to a concentration of approximately 700μmol mol‐1. No unambiguous effects of the methanol applications were detected in net photosynthesis measurements made on foliage in either of the two CO2treatments. The 75% increase in CO2, however, raised the upper‐limiting leaf temperature for positive net photosynthesis by approximately 7 C, which resulted in a 75% enhancement in net photosynthesis at a leaf temperature of 31 C, a 100% enhancement at a leaf temperature of 35 C, and a 200% enhancement at 42 C.