The constant-pressure specific heat of carbon dioxide has been measured over the range of temperatures and pressures from ambient condtions to 1000 pounds per square inch gage and 1000 degrees F using a steady-flow calorimeter operated on an open cycle. It appears that the apparatus as used in this determination will yield values with a probable error of 0.5 percent at the highest temperature level considered. The results of these tests check the widely accepted spectroscopic data within 1 percent. The values at elevated pressure are in reasonable agreement with those derived from the zero-pressure spectroscopic values and the application of the Beattie-Bridgeman equation of state. Only very limited calorimetric data are available in the literature, but substantial agreement exists with those considered reliable.