The transformation from a hemipentahydrate to a monohydrate of risedronate monosodium was studied by seed crystallization in solution. By using the in-line measurement based on ultrasonic measuring technique and the off-line measurements such as optical microscopic and crystallography, transformation of hydrate forms and crystallization of monohydrate were observed. The effect of concentration of solution and solid fraction of suspension on the ultrasonic velocity was found. The hydrate composition, concentration of solution, and supersaturation during the transformation were estimated with elapsed time. The effect of monohydrate seed, agitation rate and temperature on the induction time, transformation, and crystallization was also investigated. Eventually, transformation from hemipentahydrates to monohydrates and crystallization of monohydrate were analyzed successfully from in-line measuring technique. (C) 2011 American Institute of Chemical Engineers AIChE J, 57: 3385-3394, 2011