Technical Abstract: The LI-6400 gas exchange system (Li-Cor, Inc, Lincoln, NE, USA) has been widely used for the measurement of net gas exchanges and calibration/parameterization of leaf models. Measurement errors due to diffusive leakages of water vapor and carbon dioxide between inside and outside of the leaf chamber, and the inward dark transpiration and dark respiration released from the leaf under the gasket, can be significant. Rigorous model-based approaches were developed for estimating leakage rates of water vapor and carbon dioxide and correcting for the combination of these errors. Models were based on mass balance equations and the Dusty Gas Model for a ternary gas mixture of water vapor, carbon dioxide and dry air. Experiments were conducted using two LI-6400 systems with potato and soybean leaves. Results indicated models were reliable for estimating diffusive leakage rates of water vapor and carbon dioxide, which varied with instrument, chamber size, gasket condition, and leaf structure. A thermally killed leaf should be used for this determination. Measurement error effects on parameterization of the Farquhar et al. (1980) model as determined by curves for net photosynthesis versus intercellular carbon dioxide concentration or net photosynthesis versus mesophyll carbon dioxide concentration were substantial and each parameter had its own sensitivity to measurement errors. Results also indicated that all four error sources should be accounted for when correcting measurements.