We have measured the effects of a thermal pre-treatment of a crude oil on its pour point T-pour, and on the activation energy of a viscous flow U at 8-30 degreesC. A dramatic increase of both T-pour and U was observed in oil samples pre-heated to temperatures in the specific range from 28-30 to 39-40 degreesC. This effect may be triggered by a structural phase transitions in the oil asphaltene/resin subsystem, resulting in formation of asphaltene molecular aggregates. Below the wax appearance temperature these aggregates evidently act as cementing species in extended networks of wax crystals. The optical absorption measurements revealed an increase of a vertical concentration gradient in the samples, heated to the above specific temperatures, which may be indicative of some phase separation in the studied crude oil. (C) 2003 Elsevier B.V. All rights reserved.