The Bureau of Mines investigated modified sulfur concretes as one part of a program for utilizing sulfur in construction materials. The use of modified sulfur was studied as a means of preventing the stressing action in the concretes caused by the allotropic transformation of sulfur on solidification. Unmodified sulfur concretes have been prepared that have compression, flexural, and tensile strength equivalent or better than portland concretes. However, most of these concretes have been vulnerable to deterioration by weathering, temperature fluctuations, and freeze- thaw cycling. The sulfur was modified by reaction with dicyclopentadiene or dipentene before use as an aggregate binder. Sulfur concretes and modified sulfur concretes were prepared from acidic and basic type aggregates. Physical properties of the optimum mixtures were determined and compared with each other and with portland cement concretes. The best results were obtained by reacting sulfur with 5 pct dicyclopentadiene to prepare modified sulfur concretes. Field testing of the concrete is in progress. Present results show that modified sulfur conretes are superior to unmodified sulfur concretes and equal or better than portland concretes is compressive, flexural, and tensile strengths. Long- term aging characteristics of both modified and unmodified sulfur concretes are being determined.