Abstract This paper describes troubleshooting poor performance of a packed distillation tower and a condition called ‘upset’ in which rapid heating up occurred near the top of the tower. The investigation combined surface temperature surveys, gamma scans and gamma CAT scans, simulation and a hydraulic analysis. Temperature surveys and gamma scans revealed severe maldistribution in the top bed with vapour flowing in the central regions and liquid in the peripheral regions. Both also showed severe spraying of liquid from the reflux distributor all the way to near the tower top tangent line. There were no indications of flooding. The investigation identified feed and reflux maldistribution as the major root causes for poor performance. A simulation of test data verified poor efficiencies in both top and bottom beds. The hydraulic analysis concurred with the field tests in identifying maldistribution of feed and reflux as the root causes of poor performance. The reflux, as well as the feed maldistribution, were caused by boiling and flashing in the distributors and distributor tubes. Neither distributor was designed to handle boiling nor vapour. A major contributor was the nature of the reflux mixture, which contained a large fraction of a volatile non-key component among higher boiling light key components. This volatile non-key component readily boiled in the reflux distributor. The ‘upset’ was zonal dryout due to the distributor boiling. The maldistribution and ‘upset’ were fully eliminated by new, well-designed distributors and flashing feed inlets.