Color improvement of commercial C-9 hydrocarbon resin (c-C9HR) and prepared C-9 hydrocarbon resin (p-C9HR) has been investigated under various hydrogenation conditions over 2% Pd/-gamma-alumina catalysts. The degrees of aromatic rings hydrogenation (DHs) and molecular structure of resin were determined from nuclear magnetic resonance of H-1 and C-13 (H-1-NMR and C-13-NMR) and Fourier transform infrared spectroscopy (FTIR) analyses. The starting c-C9HR presented in yellow color (Gardner color No. 8.4). Under the hydrogenation conditions used (H-2 pressure 70 bar, 250 degrees C, and 8 h), the ethylenic proton in c-C9HR was completely removed, but the aromatic rings content remained unaltered and very little change in resin color was observed (Gardner color No.8.1). On the other hand, the starting p-C9HR contained only unsaturated aromatic proton with Gardner color No.17.1. Under similar conditions, aromatic rings in p-C9HR were converted to alicyclic rings, and its color was reduced to Gardner color No.5.7. By varying the DH of aromatics in p-C9HR, two-step decolorization was observed in which at lower DH (<10%) the color decreased sharply from 17.1 to 9.3, while further color reduction to 5.7 was obtained when the DH was increased to 94%. It is suggested that both color body and aromatic rings were the main sources contributing to C9HR color. Nevertheless, color stability of the resin during heat treatment was significantly improved by hydrogenation especially at DH > 50%. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2862-2869, 2010