In this work, we use a multi-heterodyne scanning near-field optical microscope to investigate the polarization and propagation of Bloch surface waves in an ultrathin (similar to lambda/10) ridge waveguide. First, we show that the structure sustains three surface modes, and demonstrate selective excitation of each. Then, by numerically processing the experimental data, we retrieve the transverse and longitudinal components of each of the modes, in good agreement with the calculated fields. Finally, we provide an experimental estimation of the effective indices and the dispersion relations of the modes. (C) 2010 Optical Society of America