The electronic χ ( 3 ) nonlinear optical behaviour of blue, thermally evaporated P4BCMU has been investigated at λ = 1.064 μm using TM-polarized grating coupled guided-wave techniques. The analysis involves a rigorous treatment of the electromagnetic fields in the vicinity of the grating and a theoretically generated fit to the nonlinear reflectivity response which yields both the real and the imaginary components of χ ( 3 ) in a single experiment. The values obtained are Re(χ ( 3 ) ) = - (1.3 ± 0.4) 10 - 1 0 esu and Im(χ ( 3 ) ) = (2.1 ± 0.6) 10 - 1 0 esu. Considerable care has been taken to establish the electronic origin of the effect. The results show a significant increase in two-photon absorption relative to red spun films of the same polymer at 1.064 μm.