The modification of asphalt with styrene-butadiene block copolymers and sulfur was studied to elucidate the effect of the molecular characteristics of the polymer, polymer content, and sulfur/polymer ratio on the physical properties of modified asphalts. Two types of styrene-butadiene copolymers were used (SB and SBS), which differed considerably in terms of their chain architecture, average molecular weights, and size and distribution of their polybutadiene and polystyrene blocks, as shown by gel permeation chromatography, infrared spectroscopy, nuclear magnetic resonance, and differential scanning calorimetry. Sulfur/polymer/asphalt blends were prepared by a hot mixing process and characterized by conventional tests, fluorescence microscopy, and rheology. The results revealed that the morphology of the blends is strongly dependent on polymer concentration and sulfur/polymer ratio. In-depth rheological characterization showed that the thermomechanical properties changed considerably upon addition of small amounts of sulfur. Collectively, these results suggest that sulfur increases the compatibility between polymer and asphalt by crosslinking polymer chains. Interestingly, the rheological behavior of blends prepared with a combination of SB and sulfur was similar to that exhibited by blends prepared with SBS either in the presence or absence Of Sulfur. This is explained by assuming that the addition of small amounts of sulfur to SB-modified asphalt facilitates the formation of an elastomeric network that resembles the one found in SBS-modified asphalt, effectively contributing to asphalt reinforcement. Nonetheless, the exact dosage of sulfur must be carefully controlled to prevent get formation. (C) 2009 Wiley P;eriodicals, Inc. J Appl Polym Sci 115: 3409-3422, 2010