Liquid hold-up is seen to increase as liquid viscosity and fraction of gas taken off increases suggesting corresponding increase in partial separation of phases. However, effect of liquid viscosity does not become significant until a threshold is exceeded when fraction of gas taken off equals 0.40. In all cases examined, periodicity of flow structures is observed to increase as liquid viscosity increases. Considering the results of the three investigations carried out, it can be concluded that periodicity of two-phase flow structure increases as liquid viscosity increases and transition to co-current annular flow occurs at gas superficial velocity of 21 m/s.