The dataset consists of ACi (net CO2 assimilation rate, A, versus calculated substomatal CO2 concentration, Ci) curve data from an ozone experiment during which Trifolium repens and Lolium perenne were exposed as both monocultures and two-species mixtures to an episodic rural ozone regime in large, well-watered containers within solardomes for 12 weeks. Treatments were elevated ozone (AOT40 (Accumulated Ozone Threshold exposure of 40 parts per billion) of 12.86 ppm h) or control conditions (AOT40 of 0.02 ppm h). ACi curves were carried out mid-way and at the end of the exposure period using a Portable Photosynthesis System, CIRAS (PP-Systems) to calculate the photosynthetic parameters Jmax (maximum rate of photosynthetic electron transport) and Vcmax (maximum rate of Rubisco carboxylase activity). The observed decreases in photosynthetic efficiency and capacity in elevated ozone indicate that the ability of such ubiquitous vegetation to act as a sink for atmospheric carbon may be reduced in future climates. The experiment was carried out at the CEH Bangor Air Pollution Facility. This work was funded by the Centre for Ecology & Hydrology Integrating Fund Initiative.