Leaf daytime respiration (leaf respiration in the light, R L ) is often assumed to constitute a fixed fraction of leaf dark respiration (R D ) (i.e. a fixed light inhibition of respiration (R D )) and vary diurnally due to temperature fluctuations.These assumptions were tested by measuring R L , R D and the light inhibition of R D in the field at a constant temperature using the Kok method. Measurements were conducted diurnally on 21 different species: 13 deciduous, four evergreen and four herbaceous from humid continental and humid subtropical climates.R L and R D showed significant diurnal variations and the diurnal pattern differed in trajectory and magnitude between climates, but not between plant functional types (PFTs). The light inhibition of R D varied diurnally and differed between climates and in trajectory between PFTs.The results highlight the entrainment of leaf daytime respiration to the diurnal cycle and that time of day should be accounted for in studies seeking to examine the environmental and biological drivers of leaf daytime respiration.