We have measured the nonlinear refractive index (n2) of single crystalline poly(bis(p‐toluene sulfonate) of 2,4‐hexadiyn‐1,6‐diol) (PTS) polydiacetylene channel waveguides at 1.06 &mgr;m using two different techniques. The first method employs a Mach–Zehnder interferometer to determine the intensity‐induced phase shift in the waveguide. In the second casen2is derived from the intensity‐dependent birefringence, which is measured with a Babinet–Soleil compensator. With both measurements the value ofn2in the direction of the PTS polymer chain axis is found to be ∼3×10−11cm2/W. Such a large value ofn2would allow all‐optical device applications of this material at low‐power levels.