Cellulose-based microparticles are produced by changing the pH value adjacent to the interfaces between CO2 gas bubbles and an aqueous solution containing the pH-sensitive polymer hydroxypropyl methylcellulose phthalate (HPMCP) in a simple microfluidic system. This method is inexpensive, environmentally friendly, and represents a single-step approach for synthesis of cellulose-based microparticles with a narrow size distribution.