Thermal cracking of +524 degreesC Athabasca bitumen at temperatures greater than 500 degreesC with various thicknesses of feed deposited on the reactor walls was investigated to determine if mass-transfer limitations were significant in controlling product yields. A quartz tube microreactor was used for the bitumen cracking reactions. Volatile products were removed as they evolved by constant purging of the reactor with nitrogen. Coke yields decreased from 23 to 18% with decreasing initial film thickness from. 150 to 15 mum.. A C-13-labeled tracer representative of reaction fragments from bitumen was synthesized and added to the feed in order measure incorporation of products into the coke via retrograde reactions. A 23% decrease in overall incorporation of the tracer was observed when the initial film thickness was reduced from 150 to 15 mum. A mechanism involving competition between mass-transfer of volatile compounds through the reacting liquid and retrograde reaction is proposed.