Abstract: The internally circulating reactor (ICR) has significant potential to reduce costs and simplify scale-up of pressurized chemical looping process concepts. An ICR consists of a single reactor body divided into two or three sections where different reactions are carried out. Simple ports connect the reactor sections to ensure circulation of the oxygen carrier. The cost of this simplicity is a certain amount of gas leakage between reactor sections. Quantification of this leakage revealed good CO2 purity and capture (>96%) for combustion, reforming and oxygen production chemical looping applications, but poorer CO2 separation performance for water splitting.