When gamma-Ga2O3-Al2O3 solid solutions were treated With C3H6 followed by hearing at 300 degrees C, they were colored yellow giving two bands at 340 nm (alpha band) and 430 nm (band) in their UV-vis spectra. The a and bands Were Closely related with surface tetrahedral (T-d)-Ga3+ and octahedral (O-h)-Ga3+ sites, respectively. The intensities of the two bands changed by gamma-Ga2O3-Al2O3 solid solutions prepared by various methods, although they had almost the same proportions of T-d-Ga3+ sites in the bulk structure. The intensities of the two bands can be used to diagnose the catalytic activity of gamma-Ga2O3-Al2O3 catalysts for CH4-SCR of NO. (c) 2008 Elsevier Inc. All rights reserved.