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How terrestrial biosphere models (TBMs) represent leaf photosynthesis and its sensitivity to temperature are two critical components of understanding and predicting the response of the Arctic carbon cycle to global change. We measured the effect of temperature on the response of photosynthesis to irradiance in six Arctic plant species and determined the quantum yield of CO2 fixation (phi CO2) and the convexity factor (theta). We also determined leaf absorptance (alpha) from measured reflectance to calculate phi CO2 on an absorbed light basis (phi CO2.a) and enabled comparison with nine TBMs. The mean phi CO2.a was 0.045 mol CO2 mol(-1) absorbed quanta at 25 degrees C and closely agreed with the mean TBM parameterisation (0.044), but as temperature decreased measured phi CO2.a diverged from TBMs. At 5 degrees C measured phi CO2.a was markedly reduced (0.025) and 60% lower than TBM estimates. The theta also showed a significant reduction between 25 degrees C and 5 degrees C. At 5 degrees C theta was 38% lower than the common model parameterisation of 0.7. These data show that TBMs are not accounting for observed reductions in phi CO2.a and theta that can occur at low temperature. Ignoring these reductions in phi CO2.a and theta could lead to a marked (45%) overestimation of CO2 assimilation at subsaturating irradiance and low temperature.