The aim of the present study was to investigate feedback control of a reactive crystallization process. The present study built up a control structure needed to control the driving force of reactive crystallization using the feed rate of added acid. The concentration of the crystallizing compound and pH was used to compute feedback in the closed-loop control of semi-batch precipitation. The concentration of L-glutamic acid was determined from measured MID-IR ATR-FTIR spectra based on a multivariate model. Dynamic change of set value was based on the mass of added sulfuric acid and pH. The studied properties of the product crystals were polymorphism and crystal size. The polymorphic composition was analyzed with a Raman spectrometer and was expressed by mass fraction of the a-polymorph. The obtained results showed that the developed feedback process control system allows effective control of forming of polymorphs. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 2063-2076, 2010