There are two main sources of error in the determination of Paco2 by an interpolation technique which utilizes bubble equilibrations. These are: The occurrence of metabolic acidosis during the process of storage and equilibration. Incomplete equilibration of the blood samples with the known carbon dioxide and oxygen mixture. Incomplete equilibration is particularly likely to occur if the blood has a high packed cell volume: in these circumstances there may be an underestimate of Pco2 of as much as 20 mm Hg. It is suggested that a Pco2 electrode is both more accurate and more convenient than the interpolation technique described. However, this too is subject to error and its accuracy should be repeatedly checked by tonometry.