The application of embedded radial expanding micro-channels with micro-pin fields for two phase cooling of semiconductor dies has been successfully demonstrated. Thermal data has shown effective two-phase cooling at power densities in excess of those achieved using conventional embedded parallel channel structures with similar base dimensions. An understanding of some of the basis for this improved thermal performance can be gained through detailed examination of the flow boiling characteristics in radial expanding micro-channels with micro-pin fields utilizing high speed, high resolution visualization techniques. In this work, a thermal test vehicle design incorporating manifold and thermal silicon dies as previously reported was modified to provide an optical viewing port for flow visualization. The modifications included replacing the silicon manifold die with a glass manifold die. This die was bonded to the thermal silicon die using a thin film thermoplastic rather than the solder used in the original test vehicle design. The copper manifold lid design was also modified to include a port for viewing a portion of the fluidic channels through the new glass manifold die with a high-speed video system. Thermal measurements both within the viewing area and in other quadrants of the test vehicle were available along with the high-speed video. Video obtained at up to 210 kilo-frames per second (kfps) was used to examine flow configurations, velocities and bubble growth rates. Of particular note is the extremely chaotic behavior observed at relatively high vapor qualities, behavior generally not seen in parallel channel devices without pin fields. The dynamic behaviors observed were also matched against state-of-the-art thermal models that have been shown to well predict thermal behavior in such structures.