Efficient, small, state-of-the-art passive cooling two-phase systems, i.e. advanced micro-thermosyphon cooling systems, are viable solutions for high performance datacenter servers and power electronics cooling applications. The objective of this study is to push through the "two-phase threshold" that seems to be hindering the application of this cooling technology by offering here proven experimental results (Part 1), validated steady-state and transient simulation tools (Parts 2 and 3) and a server case study (Part 4). The experimental investigation in Part 1 presents the thermal-hydraulic performance of a mini-thermosyphon loop with a small riser height, H-riser = 15.0 cm. The thermosyphon loop has a multi-microchannel copper evaporator, mounted on top of a pseudo-chip CPU emulator (heat source). Experimental results for R134a, acquired under both pumped flow and passive thermosyphon driven flow (for direct comparison) for mass flow rates up to 10 kg/hr, uniform heat fluxes, q of up to 61.4 W/cm(2) and refrigerant filling ratios up to 83% were obtained. An innovative thermal calibration method, developed as a non-intrusive mass flow measurement technique, has also been implemented to monitor the thermosyphon's operation. Summarizing in brief, the two-phase thermosyphon loop with an integrated in-line liquid accumulator offered a very sustainable cooling performance for the microchannel/pseudo-CPU package, and is a first step forward in our effort towards the integration of such two-phase passive cooling devices for data center servers and other electronic devices at heat flux of up to 80 W/cm(2) (or more).