A new type of surface site, different from those previously described, which gives rise to a paramagnetic oxygen species upon adsorption of O₂ at low temperature, has been found at the surface of crystalline quartz. This site is abundant on freshly cleaved surfaces or on chemically etched ones but absent on crystal growth faces or on samples annealed at high temperature. The abundance of this site is not related to the other surface radicals characteristic of the quartz surface originated by mechanical grinding in air. The EPR spectral features of the signal suggest an ozonide type radical O₃⁻. The adsorption of oxygen onto quartz dusts variously treated in order to modulate their fibrogenic potential in vivo indicates that a relationship may exist between the presence of this site and quartz pathogenicity. Key words: quartz, oxygen radical species, ozonide, silicosis, EPR.