Cellulose acetate is a bio-based plastic which can be used to produce foam sheets through the extrusion process. In this study, different co-blowing agents (BAs) were added to the foaming process in order to improve the extrusion foaming behavior and cell density of sheets foamed with HFO-1234ze. The effect of co-BAs on the melt viscosity was investigated with an inline extrusion rheometer. Furthermore, the resulting cell morphology of foam sheets was studied. While the plasticizing effect of the co-BA is similar to HFO-1234ze, the cell density of the foam sheets could be improved by adding co-BAs in concentrations between 0.04 and 0.16 mol/kg. Adding 0.06 mol/kg of ethanol increased the cell density from approximately 1 to 12 x 10(5) 1/cm(3) at an invariant density of approximate to 130 kg/m(3). The density reduction of the foam sheets can be calculated in good agreement from the molar concentration of the BAs. However, high amounts of co-BA slightly reduce the BA yield. POLYM. ENG. SCI., 58:E182-E188, 2018. (c) 2018 Society of Plastics Engineers