Photosynthetic gas exchange was measured in situ with either the adaxial or abaxial leaf surface illuminated on vertical, horizontal, and angled leaves of Asplenium nidus and vertical leaves of Ophioderma pendula, two epiphytic ferns in a subtropical rain forest in northeastern Taiwan. Leaves for gas exchange measurements were selected to ensure a diversity of different exposures of the two leaf surfaces to direct sunlight. For most leaves of both species, photosynthetic rates were higher when the side of the leaf that typically received more direct insolation was illuminated during the gas exchange measurement. Higher rates of net CO₂ uptake when one side of the leaf was illuminated, relative to rates when the opposite side was illuminated, were attributable to a greater biochemical capacity for photosynthesis, not to greater stomatal conductances. Based on the results of this study, the photosynthetic capacity of the two sides of the leaves of epiphytic ferns, for the most part, reflects the degree of exposure of each side of the leaf to direct sunlight, as has been found in similar studies of terrestrial taxa.