The purpose of the present experimental study is to investigate the effects of liquid viscosity on the flow patterns of upward air-liquid two-phase flow in a vertical tube of 19.2 mm in inner diameter and about 5.4 m in length. Three different liquids, including water and aqueous glycerol solutions, were employed. Kinematic viscosity of these liquids varied from 1.0 x 10(-6) to 14.7 x 10(-6) m(2)/s. The flow patterns were observed using a video recorder and still photography. The time-spatial characteristic maps of gas-liquid interfaces which were drawn using the mean liquid holdup signals detected by 70 pairs of holdup sensors arranged with the axial spacing of 15 mm over the length of 1.035 m were also used. In this report, we first defined the flow pattern of each flow. Next, the effects of liquid viscosity on the overall flow pattern and interfacial structures, or the interfacial waves, were discussed. Finally, based on those results we proposed flow pattern maps for each liquid viscosity. It is found that the flow pattern transitions strongly depend on the liquid viscosity.