The fraction of evapotranspiration (ET) attributed to plant transpiration (T) is an important source of uncertainty in terrestrial water fluxes and land surface modeling (Lawrence et al. 2007, Miralles et al. 2011). Jasechko et al. (2013) used stable oxygen and hydrogen isotope ratios from 73 large lakes to investigate the relative roles of evaporation (E) and T in ET from their catchments and to estimate global T/ET. Two recent contributions have addressed data issues with their work highlighting uncertainty propagation (Coenders-Gerrits et al. 2014) and global interception estimates (Coenders-Gerrits et al. 2014, Schlesinger and Jasechko 2014). We address here the underlying assumptions of the model applied by Jasechko et al. (2013). They assumed that the isotope ratios of the lake water incorporate spatially integrated fractionation effects of total E and T over the entire catchment. This assumption is invalid and leads to substantial errors both for catchment-scale and global estimates of T/ET.