The evaporation of an ethanol solution containing an equimolar mixture of 10-methylphenothiazine and 1,3-dinitrobenzene gave red-purple crystals. The diffuse reflection spectrum for the cocrystals showed a low reflectance from the UV through the visible spectrum until the reflectance increased at the red end of the visible spectrum. The crystal structure showed alternating π stacking of the electron-rich 10-methylphenothiazine and the electron-poor 1,3-dinitrobenzene. There were also hydrogen bonding interactions between the nitro groups from 1,3-dinitrobenzene and the aromatic hydrogen atoms from 10-methylphenothiazine. The infrared spectrum showed a shift to lower wavenumbers for the symmetric and antisymmetric stretching modes for the nitro groups. Thin films containing 10-methylphenothiazine in polystyrene were exposed to 1,3-dintrobenzene vapor, and spectroscopic ellipsometry showed an average increase in the refractive index of 0.006 through the entire range of wavelengths from 1000 to 300 nm.