Flow regimes and bubble growth are observed in a pin- fin micro- scale heat exchanger with R- 11 as the working fluid. The heat exchanger is machined in silicon and derived from a DNA micro- array consisting of 150 mu m-square fins separated by 50 mu m-square passages. The fins are staggered and oriented 45 degrees to the flow direction such that approximately 750 channel intersections occur within the volume of the exchanger. The purpose of the study is to determine if this multiply-connected geometry produces the flow blockage, reversal, and other instabilities observed in single and parallel microchannel configurations. The upper surface of the exchanger is a glass plate that provides optical access. High- speed digital photography and microscope optics are used to obtain real- time images of the flow at a framing rate of 5 kHz. The lower surface is electrically heated and instrumented with a heat flux gage. Inlet and outlet temperatures and pressures, heater and wall temperatures, and volumetric flow rate are monitored. Nucleation is observed near the entrance of the heat exchanger. In the central section, developed vapor regions are composed of broad slug- like vapor fronts immediately followed by a slowly growing bubbly flow. An annular regime dominates the downstream section of the exchanger with drop- like liquid structures appearing at the downstream edge of fins. The heat transfer coefficient decreases with exit quality as in other micro- scale exchangers; however, the flow instability present in parallel channel exchangers is not observed in this configuration.