A comparison of the effects of two types of modifiers, anthracene oil (hydrocarbon) and dioctyl phthalate (ester), on physical and mechanical properties of urethane elastomers prepared from polyetherols (propylene oxide derivatives of various functionalities and MW) and TDI was performed. The aim of the study was to assess their suitability as binders in permanently elastic urethane sealants for use in construction. The urethane elastomers under investigation were synthesized by a prepolymer method. The modifiers, added to the reactive mixture before curing, were found to be fully compatible with the urethane elastomers and did not interfere with the curing process. The usable range of application for both modifiers was found to be up to 40 phr. Over this range the modified urethane elastomers are viscoelastic liquids. The hydrocarbon modifier does enhance the hydrophobic properties of urethane elastomer, at a lower degree of crosslinking. It was found that modified urethane elastomers may be good binders for sealants, although their long-term sealing properties must be con firmed under field conditions.