Passive two-phase, micro-channel cooling for thermal management of electronics represents an efficient and viable solution to augment conventional air-cooling systems with the potential for higher power dissipation densities, increased reliability, reduced power consumption and decreased noise levels. This paper focuses on the development of a novel thermosyphon-based cooling system for electronics. The target application is a telecommunications equipment shelf unit that comprises 18 circuit pack cards. The envisioned cooling system presented here is an infrastructure-independent, air-cooled thermosyphon loop consisting of an evaporator (cold plate), manufactured with 18 individual micro-channel zones (one per circuit pack card), and connected via riser and downcomer tubes to an air-cooled condenser. The total height of the thermosyphon loop, from mid-evaporator to mid-condenser, is approximately 50 cm. The main objective of this work is to assess the effect of different condenser technologies on the thermal-hydraulic performance of the thermosyphon; prior work examining the optimal design of the evaporator, riser and downcomer tubes has already been discussed in our previously published studies. A comprehensive experimental program was conducted to evaluate two different air-cooled condensers in a front-to-back air flow configuration: a single-pass, louvered-fin, flat-tube design and a multiple-pass, wavy-fin, circular-tube design. Thermosyphon experiments were carried out with R134a as the working fluid for filling ratios from 45% to 65%, imposed uniform and non-uniform heat loads from 102 W to 1023 W, fan speeds from 7302 min(-1) to 15480 min(-1), and fan tray configurations ranging from 5 to 7 fans. Test results demonstrate that the louvered-fin flat-tube condenser provides low liquid-side frictional pressure drops and thus high refrigerant mass flow rate in the loop, which is ideal for increasing power densities as it is more conservative with respect to dry-out phenomenon. On the other hand, the multiple-pass, wavy-fin, circular-tube condenser offers low thermosyphon thermal resistances in the entire region of heat loads, primarily due to the low air-side pressure drop.