We report a method for observing and quantifying antisolvent crystallization (acetone/water system) of paracetamol in an oscillatory baffled crystallizer (OBC). With the help of optical means, paracetamol crystals passing the light sheet are recorded by a CCD camera. This allows the mass of crystals to be tracked with time and in turn the instantaneous and overall averaged crystal mass growth rates to be evaluated. By the standard sampling method, the overall averaged crystal growth rate and the percentage recovery of crystals were also determined. This work shows that there is a high correlation in data between the optical and the sampling techniques, validating the accuracy and reliability of the system. Our data are also consistent with previous work in this field. © 2011 American Chemical Society.