We developed a microfluiclic device for the investigation of crystallization kinetics from solution. The device allows to store hundreds of approximate to 100nL droplets containing a given solute and to control their temperature within 0.1 degrees C. Upon cooling, we observe independent and mononuclear crystallization events; crystal dissolution occurs as the temperature is raised. For potassium nitrate (KNO3) in water, these thermal cycles reveal the existence of two concomitant polymorphic forms. We measured, for the first time, the solubility curves of both these polymorphs, defined unambiguously the metastability extent of the solution and described why these results essentially stem from the miniaturized scale of the crystallization reactors. (C) 2008 Elsevier B.V. All rights reserved.