Experimental approaches to isolate drivers of variation in the carbon-bound hydrogen isotope composition (δ²H) of plant cellulose are rare and current models are limited in their application. This is in part due to a lack in understanding of how ²H-fractionations in carbohydrates differ between species. We analysed, for the first time, the δ²H of leaf sucrose along with the δ²H and δ¹⁸O of leaf cellulose and leaf and xylem water across seven herbaceous species and a starchless mutant of tobacco. The δ²H of sucrose explained 66% of the δ²H variation in cellulose, which was associated with species differences in the ²H enrichment of sucrose above leaf water (ε_sucrose: -126 to -192‰) rather than by variation in leaf water δ²H itself. ε_sucrose was positively related to dark respiration (R² = 0.27), and isotopic exchange of hydrogen in sugars was positively related to the turnover time of carbohydrates (R² = 0.38), but only when ε_sucrose was fixed to the literature accepted value of -171‰. No relation was found between isotopic exchange of hydrogen and oxygen, suggesting large differences in the processes shaping post-photosynthetic fractionation between elements. Our results strongly advocate that for robust applications of the leaf cellulose hydrogen isotope model, parameterisation utilising δ²H of sugars is needed.