An in situ study of the separation of a toluene-benzaldehyde mixture using a microdistillation chip with a serpentine microchannel 600 mu m wide and 400 mm long, was performed. Micropillars were incorporated along both sides of the serpentine microchannel to guide liquid flow by induced capillary force. Temperature and concentration profiles were obtained by an infrared imaging camera and in situ Raman spectroscopy. It was found that the separation occurred within a limited section of the chip, the length of which was affected by the heating and cooling temperatures. At the operating conditions studied, the separation was equivalent to 0.9-2.7 equilibrium stages, assuming total reflux. The operation of the microdistillation chip was constrained by hydrodynamics when the array of micropillars could not accommodate the liquid flow completely, thus leading to slug formation, which caused adverse effects on the separation. This work demonstrated that techniques such as Raman spectroscopy, thermal imaging and optical microscopy are efficient tools to provide insight into the operation of microdistillation, and to enable optimization of its design and operation. (C) 2012 Elsevier B.V. All rights reserved.