Evidence of crude oil stability and the tendency of asphaltene aggregation in crude oil have been investigated by absorption and fluorescence spectroscopy. Octylated asphaltenes were also used as reference because of their low aggregation tendency. Changes in the absorbance vs. concentrations in toluene solutions show that aggregation starts around 50 mg/l for Furrial asphaltene and approximately at 75 mg/l for Hamaca asphaltenes. Red shift and quenching in the fluorescence peak maximum observed for solutions when asphaltenes concentrations are increased demonstrate that the aggregation process starts at low concentration regime. These experimental results are consistent with the fact that Hamaca asphaltenes have lower tendency to aggregate than asphaltenes from Furrial crude oils. Our results verify that the tendency to form aggregates diminishes in the octylated form. q 2004 Elsevier Ltd. All rights reserved.