A lot of effort has been made to improve the understanding of the operation of loop seals. However, less is known about their influence on reactor systems. One example is the dual circulating fluidized bed reactor system consisting of two interconnected circulating fluidized beds, which are connected via loop seals. Here, the loop seals are investigated for their influence on the fluidized bed system with focus on changes in the pressure distribution and the reactor solids inventories as well as their ability to prevent gas exchange between the reactors. Detailed investigations were performed in a cold flow model using a tracer gas, mixed into the loop seal fluidization agent, which were combined with the simultaneously measured pressure profiles of the fluidized bed system. Results of experiments performed in a pilot plant were used to investigate the influence of the loop seals on the fluidized bed system under hot conditions and were compared to those obtained in the cold flow model. Investigations showed that for the loop seal, connecting the two reactors at their bottom, it is possible to influence the pressure drop in the reactors as well as the reactor solids inventories (and as a consequence the reactor solids inventory distribution) via the amount of loop seal fluidization gas. This change in the reactor solids inventories influences the solids circulation rate between the reactors as well. However, this behavior could only be recognized for low amounts of fluidization gas of the loop seal, while for higher amounts the distribution of pressure and inventories in the reactor system remained stable and unchanged. It showed that the change of pressure drop in the reactors and reactor solids inventories was caused by an increasing pressure difference between inlet and outlet of the loop seal with decreasing amount of loop seal fluidization gas. Investigations showed that this pressure difference between inlet and outlet of the loop seal was caused by the different pressure drops per height in two different zones of the loop seal (supply zone and recycle zone). Further, the maximal pressure drop, and thus the possibility to influence the reactor inventories, can be influenced by design of the loop seal (especially the height from the bottom of the loop seal to the reactors) and particle properties. Low amounts of fluidization gas of the loop seal implied an increased possibility of gas leakage between the reactors. Here, it turned out that the possibility to distribute the fluidization gas of the loop seal to different zones in the same loop seal can have significant influence. Further, the total solids inventory had as well high influence on the occurrence of gas leakage. (C) 2019 Elsevier B.V. All rights reserved.