The survival rate of extracorporeal life support (ECLS) remains overall at 60 %. Research and development is slow, partly due to the lack of sophisticated experimental models. This publication introduces a dedicated rodent oxygenator (“RatOx”) and shows preliminary in vitro classification tests. The RatOx has an adaptable fiber module size for various rodent models. Gas transfer performance over the fiber module for different blood flows and fiber module sizes were tested according to DIN EN ISO 7199.At the maximum possible amount of effective fiber surface and a blood flow of 100 ml/min, the oxygenator performance has been tested to a maximum of 6.27 ml O2/min and 8.2 ml CO2/min, respectively. The priming volume for this the largest fiber module is 5.4 ml, while the smallest possible configuration with a single fiber mat layer has 1.1 ml of priming volume. The novel RatOx ECLS system has been evaluated in-vitro to comply to a high degree with all predefined functionality criteria for rodent sized animal models. We intend for the RatOx to become a standard testing platform for scientific studies on ECLS therapy and technology.