Water that drips or splashes from a canopy or passes through it is termed throughfall. This is the first known study to examine interrelationships between throughfall isotopic fractionation and throughfall drop size. Working in a mountainous Scots pine forest, we found that throughfall splash droplets were more prevalent at the onset of rain when vapour pressure deficits were larger. This finding has important implications for water mixing in the canopy and for theories of canopy interception.