We present field observations of carbon isotope discrimination (Δ) and internal conductance of CO2 (gi) collected using tunable diode laser spectroscopy (TDL). Δ ranged from 12.0 to 27.4‰ over diurnal periods with daily means from 16.3 ± 0.2‰ during drought to 19.0 ± 0.5‰ during monsoon conditions. We observed a large range in gi, with most estimates between 0.04 and 4.0 µmol m−2 s−1 Pa−1. We tested the comprehensive Farquhar, O'Leary and Berry model of Δ (Δcomp), a simplified form of Δcomp (Δsimple) and a recently suggested amendment (Δrevised). Sensitivity analyses demonstrated that varying gi had a substantial effect on Δcomp, resulting in mean differences between observed Δ (Δobs) and Δcomp ranging from 0.04 to 9.6‰. First-order regressions adequately described the relationship between Δ and the ratio of substomatal to atmospheric CO2 partial pressure (pi/pa) on all 3 d, but second-order models better described the relationship in July and August. The three tested models each best predicted Δobs on different days. In June, Δsimple outperformed Δcomp and Δrevised, but incorporating gi and all non-photosynthetic fractionations improved model predictions in July and August.