The excessive production of reactive oxygen species (ROS), is commonly thought to be a pathogenic factor in a range of neurodegerative diseases, psychological conditions and in the etiology of aging. Traditionally, ROS have been thought undetectable in-vivo, due to their short half-life and low concentrations in living tissue. The paramagnetism of ROS may provide a means of encoding oxidative stress into MRI data. To investigate how different concentrations of ROS contribute to MRI signals, the T1 relaxivity of ROS must be determined. Using a novel method to detect ROS in-vivo, QUEST-MRI, we show that the relaxivity of the superoxide radical is in the range between 0.135-0.509 LmM-1s-1 - similar to nitroxides used as contrast agents to detect ROS in EPR.