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Summary • Will changes in nutritive values and N-relations offset initial gains in forage productivity under potential climate change observed for grass–legume pastures of south-eastern Australia? • Herbage nutritive value and symbiotic nitrogen fixation were investigated for pure and mixed swards of subterranean clover (Trifolium subterraneum ) and phalaris (Phalaris aquatica ) in field tunnels at ambient and 690 µmol mol-1 CO2 concentrations and at ambient and warmed (+3.4°C) air temperatures. • Elevated CO2 increased the nonstructural carbohydrate content of herbage whereas warming tended to decrease it. These effects were mainly on soluble carbohydrates in phalaris and starch in clover herbage. The N concentration of both species was decreased by elevated CO2 but unaffected by warming. The proportion of clover-N derived from N2 fixation was increased by 12% under elevated CO2 but decreased by 6% under warming. • Concurrent warming and high-CO2 conditions are expected to lead to improved herbage nutritive value for ruminants due to increased nonstructural soluble carbohydrate content. Longer term effects on nutritive value and N-dynamics via species persistence and competition require further study. [ABSTRACT FROM AUTHOR]