Publisher Summary This chapter reports investigations on the foamability of PETG using CO2 as the blowing agent. The use of CO2 as a blowing agent offers many advantages—it is easily available, has a relatively low critical temperature, its equation of state is accurately known, and is soluble to considerable extents in polymers. The latter allows CO2 to easily plasticize the polymer matrix, thus making it the blowing agent of choice in making microcellular foams. The solubility of CO2 in polymer melts remains appreciably high to allow production of foams with larger cells. Thus, a whole range of foams can be made using the same blowing agent. The use of CO2 in producing microcellular morphology in several polymers with relatively low Tgs has been well established. The procedure normally used is to contact the polymer with high-pressure CO2 to obtain a saturated solution. PETG is an amorphous, glycol modified poly(ethylene terephthalate) made from terephthalic acid, ethylene glycol, and cyclohexanedirnethanol by a polycondensation process. However, unlike PET, it does not undergo crystallization on heating or on plasticization by the dissolved species; the comonomer, cyclohexanedimethanol, is responsible for the completely amorphous nature of this polymer. PETG has a low Tg and high-melt strength, and has been successfully evaluated for producing medium density foamed sheets by extrusion using CO2-evolving chemical blowing agents. PETG has high affinity for CO2, and is easily processed into foams with varying cell sizes.