During reactive urethane foaming a delicate balance involving bubble growth, phase separation, and network polymerization is established under adiabatic conditions. Little progress has been reported in the measurement of rheological changes for this relatively fast (3–5 min) and highly exothermic process. A flooded parallel plate fixture was designed and adapted to follow modulus build‐up in a Rheometrics System Four rheometer. The new geometry is able to accommodate the 20–40 fold volume change observed during foaming as well as to minimize interference by the skin on the foam surface. The influence of edge effects and fixture compliance was studied by testing different cup radii and gap heights with strain and frequency sweeps on a low viscosity polydimethylsiloxane and a shaving cream foam. The evolution of storage (G’) and loss (G‘) moduli during foaming was measured on a typical slabstock foam formulation at 2.0 and 4.2 g water per 100.0 g polyol. Four regions were identified during the foaming process: 1.Bubblenucleation, within the initial 30 s of reaction; 2.Liquidfoam, a network of interacting bubbles leads to a solid‐like plateau inG’; 3.Phaseseparationwhich leads to rapid modulus rise and cell opening; and 4.Foamedelastomer, where the final (foam) modulus is reached.