Coaxial microfluidic devices were used to investigate the formation of acrylonitrile droplets and the dissolution of acrylonitrile during the droplet generation stage in a bio-hydration system. The average mass transfer coefficient of acrylonitrile was obtained using an online visual measurement method. The average mass transfer coefficient of acrylonitrile in water increased from 3.91 x 10(-3) to 11.80 x 10(-3) m/s and the droplet size decreased from 85 to 55 pm when the flow rate of the continuous phase was increased from 40 to 200 mu L/min. In contrast, the average mass transfer coefficients of acrylonitrile in a 900 U/mL free-cell solution ranged from 5.95 to 14.56 x 10(-3) m/s under the same conditions, while the bio-reaction rarely affected the size of the generated droplet. In addition, the increase of the acrylamide concentration changed the flow pattern from the dripping regime to jetting and laminar regimes and significantly reduced the droplet size. (C) 2016 Elsevier Ltd. All rights reserved.