An optical fiber, partially stripped of its cladding is shown to sense refractive index of a liquid in which the uncladded sensing region is immersed, to a high degree of precision and over a wide range of refractive index. The slope of sensor response is found to be non linear, can have either sign and can change sign at around refractive index of the fiber. The sensitivity of the sensor to refractive index change is dependent on cladding thickness and is a maximum at an intermediate thickness value. It is insensitive to the presence of absorption at the wavelength at which refractive index is being measured and to the chemical nature of the solute. Experiments designed to show that cladding modes are responsible for sensing are described.