We present the first demonstration of all‐optical phase modulation in the waveguides of an organic material {poly[bis‐(p‐toluene sulfonate) of 2, 4‐hexadiyne‐1, 6‐diol]}. The phase change as a function of optical intensity has been measured via the Mach–Zehnder interferometric method. A &pgr; phase shift has been observed for a 2‐mm‐long waveguide as the peak intensity is increased by ∼7 MW/cm2for polarization parallel to the chain axis (&lgr;=1.06 &mgr;m). Through detailed polarization‐selective measurements we have established that the observed phase change is electronic (nonthermal) in origin. The results are highly encouraging in terms of all‐optical device applications of the material.