Abstract The degree of miscibility of the various IR/BR mixtures is not obvious from inspection of the chemical structures. IR and 1,2-BR, despite their distinct differences in chemical structure, form blends that are always homogenous. On the other hand, the liquid structural differences between IR and 1,4-BR overwhelm their chemical similarities, and it is the former that governs the phase behavior. As the 1,4 content of BR increases, the thermodynamic behavior of BR/IR blends ranges from nearly ideal mixing to the occurrence of an LCST due to negative excess mixing volumes. While the observation of lower critical solution temperatures in polymer mixtures in which no specific interactions exist is unusual, the same mismatch in liquid structure gives rise to the LCST commonly observed in polymer solutions. The interaction parameter for 1,2-BR in blends with IR is circa 2×10−4, which is remarkably low given the absence of specific chemical interactions. These mixtures most closely approximate ideal mixing behavior of any known pair of chemically distinct polymers. Even mixtures of BR isotopes are more nonathermal than blends of 1,2-BR with IR; moreover, the thermodynamic behavior of these isotope mixtures are not in conformance with simple Flory-Huggins lattice theory.