Fouling of a sweet crude oil derived from oil sands was studied in a re-circulation heating loop to identify causes of refinery preheater fouling. Oil was re-circulated for periods of up to 6 days through an electrically heated annular probe. The oil was maintained under gaseous atmospheres with air/nitrogen molar ratios from zero to 0.18, at total pressures of 460 kPa. The effects of wall and bulk temperature, flow velocity, dissolved oxygen content, and mineral particle additions on the fouling rate are reported.