Microcellular polyurethane foams are generated through phase separation, which is induced by reaction-induced crosslinking or by a pressure quench. The phase separation conditions are shown to impact the microstructure of the foams. Pore growth will occur through two mechanisms : diffusion of CO2 from polymerrich regions into the pores and also through CO2 gas expansion (boiling of liquid CO2 at reduced pressure). Higher CO2 pressures for polymerization (hence, higher fluid density) provide more CO2 molecules for foaming, generate lower interfacial tension and viscosity in the polymer matrix, and thus produce higher cell densities. Increasing the functionality of the polyurethane precursors increases the T-g of the polymer network and leads to smaller cell diameters by raising the vitrification pressure and allowing less time for CO2 gas expansion to play a role in cell growth. Higher reaction temperatures result in an increase in bulk density as the cell density remains invariant and the cell size drops. The use of the more polar fluoroform as a foaming agent results in larger cells, as it is able to plasticize the polymer network and allow for gas expansion during depressurization. A constant composition method of pressure quench results in smaller cell diameters.