The ratio of the dispersed phase viscosity to that of the continuous phase is a critical parameter for microfluidic two-phase flows. Here, the influence of the viscosity ratio on liquid-liquid flow in T-shaped microchannels is studied experimentally. Three basic flow patterns, i.e., parallel, plug, and droplet flow, are observed for different sets of immiscible liquids. Flow pattern maps are plotted and generalized using a combination of Weber and Ohnesorge dimensionless numbers. In the plug flow pattern, interface deformations occur for low viscosity ratios. Existing correlations from the literature are tested against experimental data for plug velocity and lengths. Despite the significant plug interface deformations, the influence of the viscosity ratio on the plug length and velocity is negligible.