Exposure of poly(DL-lactic acid) (P DL LA), and related polymers, to supercritical CO 2 (scCO 2 ) at or below, physiological temperatures leads to very effective plasticization and liquefying of the polymers. The phenomenon arises from the high solubility and interaction of the scCO 2 in the polymer. Under these unique conditions, temperature and solvent labile molecules can be mixed efficiently into the liquefied polymer. This liquefied polymer/drug/CO 2 mixture can then be sprayed into a collecting chamber, and during this process particles of drug-loaded polymer are formed. This process is very different from rapid expansion and antisolvent based techniques that have been previously reported. In this article, we describe a method of controlling particle size during the spray process by introducing a backpressure of N 2 in the collecting chamber. This backpressure dynamically regulates the loss of CO 2 from the issuing polymer/CO 2 mixture, leading to control over sprayed particle size. In situ observation of the viscosity of the plasticized polymer indicates that a backpressure of 68 bar or greater is necessary to ensure the production of fine particles. The influences of backpressure and saturation temperature on particle size for the sprayed products are discussed in terms of observed P DL LA/CO 2 mixture viscosities.