To investigate steam-assisted gravity drainage (SAGD) mechanisms, experiments with and without carbon dioxide or n-butane mixed with steam were conducted in a scaled physical model. It is packed with crushed limestone premixed with a 12.4degrees API heavy-oil. Temperature, pressure, production data, and the asphaltene content of the produced oil were monitored continuously during the experiments. For small well separations, the steam condensation temperature and the steam-oil ratio decreased as the amount of carbon dioxide increased. The heavy oil became less mobile in the steam chamber due to lower temperatures and more viscous oil. Thus, the heating period was prolonged and the cumulative oil recovery as well as the recovery rate decreased. Less oil recovery was obtained as the fraction of carbon dioxide injected increased. Little or no change in oil recovery. and the rate of oil recovery, was observed for greater well separations regardless of the fraction of carbon dioxide in the injection gas. Similar behavior was observed when n-butane was injected along with steam instead of carbon dioxide. Cumulative oil recovery, rate of oil recovery, and steam-oil ratio decreased independent of well separation compared to a reservoir with no initial noncondensable gas.