© 2022 Elsevier LtdThis study examines the effects of the viscosity of stratified layers on liquid sloshing in a rectangular tank. It employs five different fluids to investigate viscosity in relation to free surface deformation and fluid impacts on sidewalls. A wide range of oscillation frequencies has been employed to provide resonant and non-resonant frequencies. Free surface deformations are tracked for one-layer and two-layer tests using various fluids and high-speed imaging. The experimental results have been comparatively analyzed, with a fair agreement being found between results. The viscous friction leads to energy dissipation associated with decaying pressure impacts and less free surface deformation. This phenomenon is utilized for reducing the deformation effects of one layer of fluid by adding an upper layer of fluids with a higher viscosity. The findings show the lower layer of fluid to experience a remarkable damping effect.