We experimentally investigated the effects of interstitial fluid density and viscosity on density-induced granular segregation in a rotating drum. Image processing and a particle-tracking method were used for determining the segregation index and dynamic properties of granular materials. The results indicate that the interstitial fluid density has a crucial role in density-induced granular segregation. A dimensionless factor was defined for quantifying the effect of interstitial fluid density on density-induced granular segregation. The segregation intensity increased as the interstitial fluid viscosity increased. In addition, the Stokes number associated with density-induced granular segregation was discussed in this study. (C) 2016 Elsevier Ltd. All rights reserved.