The physical and thermal properties of hand-cast polyurethane and polyurethane/polyurea elastomers prepared from prepolymers of 20 and 97% trans,trans-4,4-diisocyanatodicyclohexyl methane ( t,t-rMDI) and C 3 -and C 4 -polyethers chain extended with either 1,4-butanediol (XB) or a commercial mixture of diethyl toluenediamine isomers (DETDA) were determined. The influence of the distribution of geometric isomers of the diisocyanate and of the chain-extender building blocks on these properties is significant. Urethane/ureas are harder and have higher modulus than polyurethanes formed from the same prepolymer. The polyurethane/polyurea elastomers all have somewhat high compression set. Dynamic Mechanical Analysis (DMA) suggests that DETDA extended systems based on 20% t,t-rMDI are phase separated, as illustrated by extended rubbery plateau regions and significantly higher softening points than the corresponding XB extended ones. Uniquely, these elastomers are transparent rather than opaque as typical with most other phase-segregated elastomers. Polyurethanes based on 97% t,t-rMDI are harder and have higher modulus than those based on 20% t,t-rMDI (Desmodur W). They have good phase separation and high softening points but they are opaque. Surprisingly, there is not much difference between the physical or thermal properties of polyurethane/polyureas and polyurethanes based on 97% t,t-rMDI. Replacing XB with DETDA gives only moderate improvement in properties, but it does make the elastomers optically clear.