We present a simple, intrinsic intensity modulated fiber optic sensor for determining adulteration of petrol and diesel by kerosene. Experimental results for a prototype fabricated in the laboratory are presented. The sensing principle is based on modulation of intensity of light guided in the fiber due to change in the refractive index of the cladding formed by adulterated fuel and the phenomenon of evanescent wave absorption. The sensor is useful due to its simple construction, operation, safety with inflammable fuels and the possibility of making it compact and portable for on-road measurements.