We demonstrate how a Y-branched optical waveguide can be used for microparticle sorting. Polystyrene microparticles, optically guided in the waveguide's evanescent field, are directed down the desired, more strongly illuminated, output branch. The output of a fibre laser at a wavelength of 1066 nm is coupled to the waveguide by direct butting. The power distribution between the two output branches is selected by the relative position of the fibre to the waveguide input facet. This provides a simple method for reliable particle sorting with very high probability of success under appropriate conditions. The method can be easily combined with other particle manipulation techniques of interest for micro total analysis systems of the future.