The influence of deposition conditions on the molecular structure of thin films deposited from tetramethylsilane through plasma enhanced chemical vapor deposition was investigated using infrared spectroscopy, ultraviolet–visible absorption spectroscopy, and sputtered neutral atom mass spectrometry (SNAMS). The infrared spectra revealed a strong dependence of film structure on the deposition pressure, with high pressures (>50 mTorr) producing linear, polymeric films, and low pressures (<50 mTorr) producing amorphous, crosslinked films. The structure of the polymeric films was dominated by carbon atoms incorporated as intact, chain terminating methyl groups. The crosslinked films contained fewer methyl groups and proportionately more bridging methylene and branching methylidyne groups. SNAMS measurements revealed that the carbon‐to‐silicon ratio was inversely dependent on deposition pressure, with carbon content increasing as deposition pressures decrease. UV–visible measurements indicated that the optical properties of the films were strongly dependent on deposition pressure, particularly at pressures below 100 mTorr. Films prepared in these experiments exhibited optical gaps ranging from 2.1 to 3.9 eV. From these results, a film structure evolution model is postulated, stressing the interactions between precursor structure, fragment structure, and plasma energetics.