The ability to detect free radicals in vivo has important implications for the study of a number of physiological and pathological conditions (1–4). Two magnetic-resonance methods have been used to detect and image free radicals in vivo, namely radiofrequency continuous-wave electron paramagnetic resonance spectroscopy (CW-EPR) (5–7), and proton–electron double-resonance imaging (PEDRI) (8, 9). The former gives useful information on EPR line- widths and splittings, while the latter is able to generate high resolution images showing the distribution of free radicals in aqueous and biological samples. We have recently developed an instrument capable of sequential CW-EPR and PEDRI experiments on the same sample, combining the advantages of both methods. The switchover time between the two modes of operation is about five seconds. In this Note, we obtain much higher spatial resolution than EPRI, because give details of the apparatus and present our first results with aqueous samples.