We investigated viscosity dependence of sonoluminescence (SL) in aqueous solutions of glycerol at acoustic frequencies of 50 and 150 kHz. The concentration dependence of SL intensity showed opposite behaviors at the two frequencies. At 50 kHz, the SL intensity increased with increasing glycerol content taking a maximum at 80% glycerol. At 150 kHz, the SL intensity decreased monotonically with increasing glycerol content. Highspeed shadowgraphy of cavitating bubbles with a frame speed of 1 Mfps demonstrated the difference in bubble dynamics at the two frequencies, and clarified the observed results of SL intensity. The increase in SL intensity at 50 kHz resulted from the increase in the number of bubbles exhibiting spherical oscillation caused by the high viscosity. The decrease in SL intensity at 150 kHz resulted from the small expansioncompression ratio of bubble oscillation caused by viscous damping. A numerical simulation of bubble oscillation supported the results.