A common injector geometry upstream of a static mixer is the centerline injector. A flow instability. can arise due to viscosity differences between the injected core-flow and, the-outer co-flow. This, instability can adversely affect the,effectiveness of the mixing operation. An experimental, investigation of miscible, viscosity-stratified flow, in a circular geometry, was performed, using, Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV). The experimental results for the stable region,agree with the analytical results. The unstable region exhibits different modes depending on the viscosity ratio, volume flux, ratio, and Reynolds number. The modes include wavy core-flow with fissures and wavy, core-flow with core breakup. The time-averaged experiment velocity profiles for the unstable core. indicate a broadening,of the jet at the centerline, which is consistent with the LIF visualization.