Intermodal diffusion in a large-diameter plastic-clad silica step-index fiber is measured with a near-field projection of the modal pattern onto a CCD camera. The diffusion process is found to obey a 1/θ(2) dependence (where θ is the axis-crossing angle of the mode). The diffusion constant is estimated to be 1.3 × 10(-5) rad(2) m-(1). The contribution of mode mixing to evanescent-wave spectroscopy in such fibers is discussed. When all bound meridional modes are launched, the effect of diffusion is a small reduction- some 10% maximum-in the fraction of evanescent-wave power outside the core for the 0.4 numerical aperture fiber used.