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• We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of leaf cellulose (δ18Ocellulose) in a drought-prone, temperate grassland ecosystem. • A new allocation-and-growth model was designed and added to an 18O-enabled soil–vegetation–atmosphere transfer model (MuSICA) to predict seasonal (April–October) and multi-annual (2007–2012) variation of δ18Ocellulose and 18O-enrichment of leaf cellulose (Δ18Ocellulose) based on the Barbour–Farquhar model. • Modelled δ18Ocellulose agreed best with observations when integrated over c. 400 growing-degree-days, similar to the average leaf lifespan observed at the site. Over the integration time, air temperature ranged from 7 to 22°C and midday relative humidity from 47 to 73%. Model agreement with observations of δ18Ocellulose (R² = 0.57) and Δ18Ocellulose (R² = 0.74), and their negative relationship with canopy conductance, was improved significantly when both the biochemical 18O-fractionation between water and substrate for cellulose synthesis (ϵbio, range 26–30‰) was temperature-sensitive, as previously reported for aquatic plants and heterotrophically grown wheat seedlings, and the proportion of oxygen in cellulose reflecting leaf water 18O-enrichment (1 – p ex pₓ, range 0.23–0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. • Understanding physiological information in δ18Ocellulose requires quantitative knowledge of climatic effects on p ex pₓ and ϵ bio.