Measurement of CO2/H2O vapor relationships for crop canopies provides valuable information about the relationship between two plant processes, i.e., carbon uptake and transpiration. These measurements have been difficult in the past because of the need to have two different measurement systems. Development of the fast response CO2/H2O vapor systems coupled with sonic anemometers and surface temperatures derived from infrared thermometers provide new opportunities to explore these relationships. Observations have been made over corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) canopies since 2000 using energy balance systems and fast response CO2/H2O vapor sensors. These units are deployed over these canopies throughout the complete growing cycle. These measurements are most valid when there is complete canopy cover and the soil surface is dry so that the plant canopy is the dominant source and sink in the CO2/H2O vapor exchange. The slope of the relationship is mg CO2/J H2O vapor and this slope changes throughout the day and during drydown periods and represents canopy level water use efficiency (WUE). In corn canopies, WUE in the morning was -0.01 mg/J while in the afternoon was reduced to -0.006 mg/J. We observed that there was a larger change in CO2 values during the afternoon than transpiration values suggesting that the canopy is becomes less efficient in CO2 uptake. During a dry down period the slope of the line was relatively unchanged in the morning while the afternoon values declined to -0.003 mg/J. Similar relationships were found for soybean canopies with WUE values of -0.003 mg/J in the morning and -0.002 mg/J in the afternoon. During drydown periods the soybean canopies showed a similar relationship as the corn. These techniques can easily provide a more complete understanding of changes in WUE in response to management practices and refine our understanding of genetic-environment interactions.