Leaf conductance gL is strongly influenced by environmental factors like CO2, irradiance and air humidity. According to Ball et al. (1987), gL is correlated with an index calculated as the product of net CO2 exchange rate A and ambient water vapour concentration WA, divided by ambient CO2 concentration cA. However, this empirical model does not apply to high values of gL observed at cA below CO2 compensation concentration Γ. Therefore, we applied modified indices in which A is replaced by estimates for the rate of carboxylation. Such estimates, P1 and P2, were determined by adding to A the quotient of Γ and the sum of gas phase resistance rg and intracellular resistance for CO2 exchange ri, P1 = A+Γ/(rg + ri), or the quotient of Γ and ri, P2 = A + Γ/ri. If P2 is chosen, cA in the Ball index has to be replaced by the intercellular CO2 concentration ci. By using the modified indices P1ċWA/cA and P2ċWA/ci, we analysed data from the C3 species Nicotiana tabacum and Nicotiana plumbaginifolia, the C3–C4 intermediate species Diplotaxis tenuifolia, and the C4 species Zea mays. The data were collected at widely varying levels of irradiance and CO2 concentration. For all species uniform relationships between gL and the new indices were found for the whole range of CO2 concentrations below and above Γ. Correlations between gL and P1ċWA/cA were closer than those between gL and P2ċWA/ci because P1/cA implicitly contains gL. Highly significant correlations were also obtained for the relationships between gL and the ratios P1/cA and P2/ci.