The evolution of (micro)phase separation during the bulk formation of a copolyurethane has been followed by small-angle X-ray scattering (SAXS). The material examined comprised hard segments formed from 4,4′-diphenylmethane diisocyanate and 1,4-butanediol and soft segments based on a poly-(oxyethylene-block-oxypropylene) diol of molar mass 2000. A small reaction injection molding machine was used to meter, mix, and inject stoichiometric amounts of reactants into a thermostated cell, fixed to an optical bench on beam line I-IV at the Stanford Synchrotron Research Laboratory. A full scattering pattern was collected every 0.7 s. Generally, higher polymerization temperatures led to a faster rate of (micro)phase separation but a lower ultimate degree of (micro)phase separation. An induction time was observed that decreased with increasing temperature. The scattered intensity at fixed angle increased with the square of time, and the length scale in the material increased with temperature. However, it was not possible to distinguish whether this structure was due to hard segment crystallization or noncrystalline microphase separation in this system.