The crystal size distribution (CSD) of potassium dihydrogen phosphate (KDP) and inclusion ratio of mother liquor have been measured in a mixed suspension mixed product removal (MSMPR) crystallizer. A small amount of sodium dihydrogen phosphate (SDP) was added as an impurity tracer in order to measure the inclusion ratio of mother liquor inside the KDP crystals. The effects of residence time on the CSD and inclusion of mother liquor were investigated. The crystal growth rate and nucleation rate were determined from the population density balance theory by assuming a size-dependent function of the growth rate, because the size-dependent growth rate of KDP crystals was observed under a microscope. MSMPR operation with a shorter residence time produced larger KDP crystals, and showed more size-dependency of the growth rate. The inclusion ratio was depicted as a function of crystal size, growth rate, and agglomeration. The degree of agglomeration was effectively correlated with the inclusion ratio of the KDP crystals that had a large hardness property. (C) 2004 Elsevier B.V. All rights reserved.