A series of flow visualization experiments was carried out to examine the pore scale behaviour of solution gas drive process in heavy oil reservoirs. The main objective was to test several speculative theories that have been put forward to explain anomalous production behaviour of heavy oil reservoirs producing under solution gas drive process. Contrary to previous postulations, the asphaltene constituents did not appear to play a significant role in nucleation and stabilisation of the gas bubbles evolved during the solution gas drive process. Experimental evidences also suggest that the production of heavy oil is not accompanied by a large population of micro-bubbles. These observations suggest that the production enhancement in solution gas process in heavy oil reservoirs is related to other mechanisms such as viscous coupling effects, sand production, wormhole effects, etc.