Chemical looping is a technology where the needed for combustion oxygen is provided by an oxygen carrier. It offers many advantages such as prospective carbon dioxide sequestration-ready stream, and nitrogen oxides emission significant reduction. Thermogravimetric analysis (TGA) allows to determine properties of materials in the different atmospheres (neutral, oxidizing, reducing) which is critical for oxygen carriers. An Oxygen carrier (OC) is a material (usually d-block metal oxide) used as a medium delivering oxygen. OC should show out among others an acceptable oxygen transport capacity, and a good reactivity toward fuel and air, as well as a good mechanical strength above 1 N. Ability of spontaneous releasing oxygen, so called CLOU effect, is desirable especially for fossil fuels combustion speeding up the process. In this paper iron-manganese mixed with different additions were tested using TG to determine their reactivity toward selected fuels, and a special attention was paid to obtaining CLOU properties, which were also profoundly evaluated. Acknowledgement: The work was financed from the National Science Centre, Poland Project No. 2020/37/B/ST5/01259.