Using the broadening of the electron paramagentic resonance (EPR) linewidth of paramagnetic particles by oxygen, it is possible to make measurements of the partial pressure of oxygen in vivo. While the results obtained so far with EPR oximetry are very encouraging, several paramagnetic materials may lose their responsiveness to oxygen in tissues. This aim of this study was to provide evidence that an appropriate coating can preserve the oxygen sensitivity of paramagnetic materials in vivo. Two charcoals that have the oxygen-sensing properties required for EPR oximetry (combined with a tendency to lose responsiveness to oxygen when placed in tissues) were coated using pyroxylin. Sensitivity to variations in pO2 was checked by inducing hypoxia in the muscles of mice injected with charcoal. While the uncoated material lost responsiveness to oxygen within few days, the particles coated with 20-30% of pyroxylin did not lose their responsiveness for more than 2 months.