Microstructured optical fiber (MOF) sensors are nowadays utilized or proposed for the measurement of a large variety of physical and chemical parameters, e.g. in environmental and biological applications because of their efficiency and versatility. This paper, at first, gives a short review of MOF sensors reported in literature. An evanescent field MOF sensor designed and refined with respect to the contaminant to be detected is illustrated, too. The sensor behaviour is simulated for different MOF cross section geometries, hole distributions, polymeric claddings. The optimal MOF configuration which maximizes the absorbance is identified.