Flow maldistribution causes declining plate-fin heat exchanger thermal-hydraulic performance. A first-of-its kind experimental facility and the related data acquisition system were constructed for studying liquid gas flow distribution in a plate-fin heat exchanger. The gas Reynolds numbers ranged from 1880 to about 2600 and the inlet dryness (i.e., quality) from 12% to 41%. Two-phase flow maldistribution among the heat exchanger passages was more widespread compared to that of single-phase flow. More specifically, the liquid-phase distribution was more uneven compared to the gas-phase distribution. The inlet flow rate and dryness were identified as the chief factors affecting the distribution of phases in the heat exchanger. For a given inlet dryness, the two-phase flow distribution became increasingly non-uniform with the inlet gas flow rate, consistent with the behavior observed for singlephase flow. Additionally, the non-uniformity in the gas flow distribution decreased and that in the liquid flow non-uniformity increased with increasing inlet dryness fraction. A novel distributor design, with a complementary fluid cavity was also built and tested. Experimental results show that improving the distributor design is very effective in improving the two-phase flow distribution in plate-fin heat exchangers. Based on heat transfer studies conducted at a single Reynolds number of about 1500 and a dryness of 29.2%, the heat exchanger effectiveness was also correlated as a function of the dryness distribution non uniformity parameter S. The effectiveness was found to reduce as the flow distribution became more uneven, highlighting the importance of accounting for and controlling the flow maldistribution through proper distributor design. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.