The method of additive properties was applied to the density and glass transition temperature, T-g, Of linear amorphous polyurethanes. This method assumes that certain polymer properties result from the additive effects of unique constituent group properties and that these group properties are independent of their environment. To determine the component properties, 14 model polyurethanes were synthesized. The polymers contained only the following four groups: urethane, phenylene, methylene, and ether oxygen. The densities and T-g＇S of these polymers were measured. Using these measured values, the known composition of each polymer, and an appropriate additive form, the molar component properties were calculated using a matrix least-squares simultaneous fit algorithm. An error analysis confirmed that the component values were determined with greater accuracy than literature results. The densities ranged from 1.0 to 1.2 g/cm(3) and measured versus predicted densities agreed to within 0.3% on the average. T-g＇S ranged from -70 degrees C to 70 degrees C and measured versus predicted T-g＇S agreed to within 1.3% on the average.