This paper compares transpiration efficiency (TEc = shoot biomass corrected by seed oil-synthesis cost/ transpiration per unit vapour pressure deficit) of standard-height and semi-dwarf sunflower (Helianthus annuus L.) crops. Three irrigation regimes that differed in timing and amount of irrigation were applied. Transpiration was calculated as the difference between evapotranspiration (assessed by hydrological balance) and soil evaporation (measured using evaporimeters). Irrigation treatments and irrigation × cultivar interaction did not affect TEc. Across irrigation regimes, seasonal TEc was 4.70 and 3.64 g m-2 mm-1 kPa (P < 0.003) for the standard-height and semi-dwarf crops, respectively. Seasonal differences in TEc arose from differences during the post-anthesis period. The lower post-anthesis TEc of the semi-dwarf cultivar was related to a smaller radiation-use efficiency. Possible causes for the differences in post-anthesis radiation-use efficiency are discussed.