The paper presents a comparison of transient calculations for two 80-MWt MOX-fuelled experimental accelerator-driven systems (XADS), one cooled by lead-bismuth eutectic and the other by helium. The results for protected (with accelerator beam trip) and unprotected (without accelerator beam trip) transient overpower, spurious beam trip, loss of flow, loss of heat sink, and loss of coolant accidents, as well as the failure of heat exchanger tubes, are analysed for the two systems. The analysis was performed using TRAC/AAA, which is part of the PSI FAST code system. The advantages and shortcomings of the two designs from the viewpoint of their transient behaviour are discussed. [All rights reserved Elsevier]