Observations of oscillations in bending loss as a function of illuminating wavelength in monomode optical fibers are presented. A model is developed based on the interference between the core-guided mode, and a 'whispering gallery' (WG) mode established by reflection at the buffer-air interface. It is shown that the relative phase of the modes is a function of the dimensions and refractive index of the buffer coating. The phase is determined in experiments where the bend radius and wavelength of illumination are varied, allowing the buffer diameter and refractive index to be determined by a best fit to experimental data. The practicality of this method for the characterisation of buffer properties is discussed.