One method of preparing a primary reference gas mixture is the gravimetric blending method. Uncertainty of a few mg in mass measurements is unavoidable when preparing reference gas mixtures under current laboratory conditions with our facilities, equipment, and materials. There are many sources of errors when using this method. In this study, several sources of errors were re-evaluated for our process for preparation of carbon dioxide in synthetic air. As a consequence of the re-evaluation, it was found that some sources of errors had significant effects on gravimetric concentrations of the gas mixtures. These sources are: (1) different masses of the reference cylinder and sample cylinder (an error in the readings of the electronic mass comparator), (2) leakage of the inner gas from valves of the cylinders, and (3) cooling of the gas cylinder caused by filling with high-pressure liquefied carbon dioxide gas. When the mass measurements were performed under uncontrolled conditions, the errors due to sources (1), (2), and (3) were as high as 20 mg, 24 mg, and 13 mg, respectively. In this paper, the detailed results from re-evaluation of these sources of errors are discussed.