The effect of pH and supersaturation on the growth kinetics of the pyramidal (101) and prismatic (100) faces of KDP crystals is reported. Growth rates of these faces at the microscopic level are measured using the laser-polarisation-interference technique, and a comparative analysis is carried out. In the light of these results, an attempt has been made to clear the ambiguity surrounding the effect of pH on the growth rates of KDP crystals along the a- and the c-axis. We have found that the growth rate of (101) as well as (100) face is maximum at stoichiometric pH of 4.2 (at all supercoolings considered), and it decreases on either side of this value. This is in agreement with the results of Efremova et al. (Crystallogr. Rep. 38(5) (1993) 674), and contradictory to those of Wang et al. (J. Crystal Growth 83 (1987) 471), Marecek et al. (Kristall Technik 4(1) (1969) 39) and Rashkovich et al. (J. Crystal Growth 151 (1995) 145) where it is reported that growth rate of (100) face increases if we go above the stoichiometric pH. For performing these on-line and in situ studies, a novel crystal-growth cell, a special seed-orienting mechanism for mounting the desired crystal face in the laser beam path, and a data acquisition system were developed. The shadowgraphy technique was employed for making an accurate measurement of the saturation temperature. (C) 2002 Elsevier Science B.V. All rights reserved.