New sulfur-containing aromatic diamines with methyl groups at the ortho position of amino groups have been developed to prepare highly refractive and transparent aromatic polyimides (Pls) in the visible region. All aromatic Pls derived from 4,4'-thiobis[2 ''-methyl-4 ''-(p-phenylenesulfanyl)aniline (2), 4,4'-thiobis[2,'' 6 ''-dimethyl-4 ''-(p-phenylenesulfanyl)aniline (5), and aromatic dianhydride, 4,4'-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (6) were prepared via a two-step polycondensation. All Pls showed good thermal properties, such as 10% weight loss temperature in the range of 497-500 degrees C and glass transition temperatures above 196 degrees C. In addition, the Pls showed good optical properties, such as optical transparency above 75% at 450 nm with a 10-mu m film thickness, high refractive indices ranging from 1.7135 to 1.7301, and small in-plane/out-of-plane birefringences between 0.0066 and 0.0076. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 656-662, 2010