The Picarro G1101-i and G2101-i gas analyzers provide field measurements of the stable isotope ratio d13CO2 with a precision of 0.3 permil in five minutes. In an isotope ratio mass spectrometry (IRMS) laboratory, it is customary to dry the samples to very low dew points (< - 45C) prior to isotope analysis. For field work, however, drying the samples is in many cases inconvenient or impractical. Water vapor can affect the reported delta via spectroscopic broadening or direct spectroscopic interference. In the G2101-i, the effects of the water vapor concentration, including its isotopes, has been characterized, and the reported isotope ratios have been corrected. Residual errors of less than 0.3 permil can be achieved in the G2101-i, even in very humid gas streams.