We present an easy-to-use microfluidic set-up, easily transferable to the laboratory and determine an accurate method for metastable zone width measurement. We clearly define a zone in the phase diagram where nucleation is mononuclear. We nucleate a single crystal of metastable phase, which turns out to be stable. This approach holds promise for the control and the study of crystallization processes. (c) 2010 Elsevier B.V. All rights reserved.