Canister desorption is a widely used technique to measure the gas content of coal. The gas content data, when normalized to volume/weight and multiplied by coal seam mass, is used to estimate the gas in place in an area around the cored hole. However, the gas content and the percentage of each constituent are likely to be influenced by trapped air in the canister at the time of the coal enclosure and subsequent sealing. Freshly-cored coal samples were collected from three mines, mining coal from Bulli seam, Sydney Basin. The studied underground mines were Appin West, West Cliff, and Tahmoor. The research programme, spanning for a period of four years, focused only on the influence of the trapped air in the canister on the coal gas percentage of each constituent. It was found that the percentage of each coal gas constituent was influenced by the trapped air in the canister space. The effect of trapped air was extended to the component percentage of the gases in the crushed coal samples, normally used for the estimation of Q3.