Small-diameter metallic single-wall carbon nanotubes (SWNT) were separated from larger diameter semiconducting SWNT when CoMoCAT samples (supplied with abundance of (6,5) species) were left standing at ambient laboratory condition for a few days in dimethylformamide solution containing ethylanthraquinone (EAQ). SWNT enriched in larger diameter semiconducting species by this method were used to fabricate SWNT network-based thin film transistors (TFTs). The resulting devices had mobilities of 0.2 cm2/(V s) and on/off ratios of about 104. The on/off ratios were greatly improved, by approximately 2 orders of magnitude, over those of TFTs made with pristine SWNT. The enrichment in semiconducting tubes was corroborated by UV−vis−NIR absorption, photoluminescence excitation (PLE), and resonance Raman scattering spectroscopy. Density functional simulations show that hydroxyl radicals preferentially attack small-diameter metallic and then small-diameter semiconducting nanotubes over larger diameter semiconducting tubes. We propose that EAQ forms radicals in ambient lighted conditions, resulting in this diameter- and metallicity-selective reaction which increases the density of the target species, promoting their sedimentation under centrifugation. This simple method of obtaining semiconducting-enriched SWNT samples should be widely applicable for printed electronic devices.