A novel type of integrated optical sensor has been evaluated theoretically. The sensing is based on the large effect of the refractive index of the measurand (generally a liquid) to the mode profile of a guided mode close to the cut-off. A special type turns over into a leaky mode-based sensor. The sensor has small size and is simple to fabricate. For the SiON technology-based structures used as a vehicle for the evaluation, a resolution of the refractive index in the range of 10(-6) RIU has been achieved; however, with an operation range around a certain working index of approximate to 2 x 10(-4) RIU only. This working index can be shifted to any desired value, within a certain range, by appropriately choosing the geometrical and material parameters. Because of technological tolerances, for practical operation, read-out based on wavelength scanning is required. The sensor can be used especially as an alarming sensor. (C) 2014 Elsevier B.V. All rights reserved.