The effect of ultrasonic irradiation on the behavior of primary nucleation of L-Arginine (L-Arg) and L-Serine (L-Ser) in supersaturated solutions at 30 degrees C was studied. The induction period for both amino acids increased with increasing ultrasonic irradiation energy up to a certain degree and subsequently decreased. These results indicate that a low degree of ultrasonic irradiation can inhibit primary nucleation rates, whereas a high degree of ultrasonic irradiation can enhance them. Measurements of crystal size for each irradiation and supersaturation condition showed that mean particle size is affected by ultrasonic irradiation and is highly correlated with the induction period. As a result, the desired crystal size of the final product is expected to be obtained by optimally controlling the conditions of supersaturation and ultrasonic irradiation. (C) 2009 Elsevier B.V. All rights reserved.