Two-phase micro-channel heat sinks are prime candidates for incorporation into thermal control systems (TCSs) of future space vehicles and planetary habitats. Unlike small heat sinks employed in the electronics industry, TCS heat sinks are characterized by large length-to-diameter ratio, for which limited information is presently available. This study employs a 609.6-mm long by 203.2-mm wide heat sink containing 100 of 1 x 1 mm(2) micro-channels and uses R134a as working fluid. The large length-to-diameter ratio of 609.6 is especially instrumental to capturing detailed axial variations of flow pattern and corresponding variations in local heat transfer coefficient. High-speed video analysis of the inlet plenum shows appreciable vapor backflow under certain operating conditions, which is also reflected in periodic oscillations in the measured pressure drop. In fact, the backflow frequency captured by video matches closely the frequency obtained from Fourier analysis of the pressure drop signal. While density-wave oscillations are encountered in individual channels, the phenomena observed are more closely related to parallel-channel instability. It is shown the periodic oscillations and vapor backflow are responsible for initiating intermittent dryout and appreciable drop in local heat transfer coefficient in the downstream regions of the channels. A parametric study of oscillation frequency shows a dependence on four dimensionless parameters that account for amount of vapor generation, subcooling, and upstream liquid length, in addition to Weber number. A new correlation for oscillation frequency is constructed that captures the frequency variations relative to these individual parameters. (C) 2017 Elsevier Ltd. All rights reserved.