Many methods are available for the measurement of dissolved carbon dioxide in an aqueous environment. Standard titration is the typical field method for measuring dissolved CO₂ in aquaculture systems. However, titrimetric determination of dissolved CO₂ in marine water aquaculture systems is unsuitable because of the high dissolved solids, silicates, and other dissolved minerals that interfere with the determination. Other methods used to measure dissolved carbon dioxide in an aquaculture water included use of a wetted CO₂ probe analyzer, standard nomographic methods, and calculation by direct measurements of the water's pH, temperature, and alkalinity. The determination of dissolved CO₂ in saltwater based on partial pressure measurements and non-dispersive infra-red (NDIR) techniques with a CO₂ gas analyzer are widely employed for oceanic surveys of surface ocean CO₂ flux and are similar to the techniques employed with the head space unit (HSU) in this study. Dissolved carbon dioxide (DC) determination with the HSU using a infra-red gas analyzer (IRGA) was compared with titrimetric, nomographic, calculated, and probe measurements of CO₂ in freshwater and in saltwater with a salinity ranging from 5.0 to 30 ppt, and a CO₂ range from 8 to 50 mg/L. Differences in CO₂ measurements between duplicate HSUs (0.1–0.2 mg/L) were not statistically significant different. The coefficient of variation for the HSU readings averaged 1.85% which was better than the CO₂ probe (4.09%) and that for the titrimetric method (5.84%). In all low, medium and high salinity level trials HSU precision was good, averaging 3.39%. Differences existed between comparison testing of the CO₂ probe and HSU measurements with the CO₂ probe readings, on average, providing DC estimates that were higher than HSU estimates. Differences between HSU and titration based estimates of DC increased with salinity and reached a maximum at 32.2 ppt. These differences were statistically significant (P < 0.05) at all salinity levels greater than 0.3 ppt. Results indicated reliable replicated results from the head space unit with varying salinity and dissolved carbon dioxide concentrations.