Results from four field based collaborative tests and from one laboratory based collaborative test of the Orsat analytical procedure are discussed. The results from the five collaborative tests demonstrate that routinely using Orsat data to convert particulate emissions from combustion sources to the reference conditions of 50% excess air and 12% carbon dioxide may introduce sizeable errors in the corrected particulate loading. Ways to improve the Orsat apparatus and increase the reliability of the data are suggested. Also reported are the results from field and laboratory studies on the reliability of using individual carbon dioxide and oxygen analyzers of the Fyrite type to determine stack gas molecular weight. The laboratory study, which was done using three cylinders containing mixtures of carbon dioxide, oxygen, carbon monoxide, and nitrogen of known concentration, determined that these analyzers give carbon dioxide and oxygen analyses of sufficient reliability to yield accurate molecular weights. The results of the field studies, which were done on actual flue gas samples, also support this conclusion.