Exciton surface polaritons (ESP's) have been observed by the technique of attenuated total reflection (ATR) spectroscopy at 295 K on the (100) face of the polymerized diacetylene crystal poly-2,4-hexadiyne-1,6-diol bis (p-toluene sulfonate). The ATR experiments were performed by measuring the p-polarized reflectivity as a function of angle of incidence at constant wavelength. By changing the wavelength of the exciting light and repeating the angular scans, the experimental dispersion of the surface mode was determined for the wavelength range 632.8 to 596 nm. The intensity of the electromagnetic field in the three regions, prism, gap and sample, was calculated to demonstrate the localization of the ESP at the crystal surface. © 1978.