This paper presents an investigation of the morphology of growing polyurethane (PU) rigid foams during the very first seconds of the process by cryogenic scanning electron microscopy (cryo-SEM) performed at -150 degrees C. The heterogeneous nature of the initial mixture has been revealed with the presence of submicron size physical blowing agent droplets (isopentane), mu m size dispersed phase nodules, and large air bubbles dispersed in a continuous matrix. Following the evolution of the microstructure during foaming by cryo-SEM suggested that the isopentane liquid droplets (undissolved part of the physical blowing agent) did not vaporize to create their own bubbles. These observations were confirmed by showing that the number of air bubbles per unit volume (similar to 10(6) bubble/cm(3)) was similar to the cell population density of the final foam (similar to 10(6) cell/cm(3)), while the number of isopentane droplets initially present was found to be six orders of magnitude higher (similar to 10(12) droplet/cm(3)). This all means that isopentane molecules initially dissolved in the continuous phase diffuse into the pre-existing air bubbles with no energy barrier to overcome (non-classical nucleation) whereas the isopentane droplets simply act like reservoirs. Finally, despite our best efforts, there is still some doubt whether polymeric 4,4-diphenylmethane diisocyanate (PMDI) is dispersed in the polyol phase, or polyol dispersed in PMDI. (C) 2019 Elsevier Inc. All rights reserved.