Results are presented from third harmonic generation, (THG), experiments on a number of polysilane films. The polymers studied include poly(di-n-hexylsilylene) PDHS, and copoly(di-n-hexyl, hexylmethylsilylene) PDHS / PHMS. These polymers exhibit positive thermochromism, where the transition temperature and spectral properties are dependent on the copolymer ratio. The nonlinear susceptibility, , in the low temperature phase of the polymer increases by approximately 4 − 5 times that of the high temperature phase where the absorption maximum decreases by ∼ 60 nm. The polymers are prepared as neat, solid films on fused silica substrates. Thickness and refractive index measurements of these polymers were made by UV-VIS-NIR spectroscopy.