An investigation has been undertaken to examine the stability of a gas-liquid interface in a vertical cylindrical vessel subjected to a sinusoidal vertical motion. Experimental results indicate that there is a certain limiting critical frequency at which the gas-liquid interface becomes unstable, significant amounts of liquid are thrown vertically from the interface and the interface thus loses its coherent structure. Experimental data are compared with analytical solution based on the reduction of governing non-viscous equations to Mathieu equation, and good agreement between theory and experiment is found.