The effect of dimensionality on the photobleaching characteristics of Si-based polymers with phenyl substituents was investigated by studying the change in absorption upon exposure to 248- and 337-nm excitation. Poly(methylphenylsilylene) was chosen as an archetype of the linear-chain polysilanes, and poly(phenylsilyne) was used as the comparable network polysilyne. Differences in bleaching behavior of the two types of polymers are explained in terms of Si-Si bonding and electronic interactions between the phenyl groups and the silicon frameworks. Wavelength-dependent effects include the degree of photodegradation of the phenyl rings and changes in polymer configuration due to photoscission and crosslinking. Absorption changes were accompanied by changes in the refractive index of the exposed regions of the samples. The refractive index modification was used to write embedded strip waveguides in the photosensitive thin films.