We describe the synthesis of mixed, or binary, polymer brushes containing polystyrene (PS) and poly(methyl methacrylate) (PMMA). These films were synthesized by a grafting-from (GF) strategy from gold substrates that were coated with a bis-thiol free radical photoinitiator, similar to 2,2'-azobis-(isobutyronitrile) (AIBN). The PS layer was 70 nm thick for all substrates, and the PMMA was varied up to 700 nm. The number-average molecular weight (M.) of the ungrafted polymers was determined by gel permeation chromatography (GPC) and multiangle light scattering (MALS); the M-n for bulk PS and PMMA was similar to57 000 and similar to515 000 Da, respectively. The air/liquid interface was probed by static water contact angles (theta) and X-ray photoelectron spectroscopy (XPS). We found that both theta and the oxygen concentration varied depending on whether the substrate was immersed in a nonselective solvent (tetrahydrofuran) or selective solvent for PS (cyclohexane) or PMMA (isobutanol). The morphology was monitored by atomic force microscopy (AFM), and we observed a dimple-type structure in cyclohexane and isobutanol, with less nanophase separation after immersion in tetrahydrofuran. Reflection-absorption infrared spectroscopy (RAIRS) was used to quantify the % PMMA in the binary brushes, which ranged from 0 to 90%, and was confirmed by XPS. These substrates exhibited reversible switching, and both lateral and layered nanophase separation were observed. The elasticity of a mixed brush was probed by AFM and was found to lie between that of neat PS and PMMA.