Triplet state NO dimers stabilized on the surface of sulfated zirconia (g(perpendicular to)= 1.993, g(parallel to) = 1.942, D-perpendicular to = 195 G, E approximate to 0) have been observed by ESR. They are formed by adsorption of NO molecules on pair (acceptor) surface sites represented, most likely, by two adjacent (d=5.2 Angstrom) low-coordinated Zr4+ ions. A new single line (Lorentz line with g = 1.993 and line width 18 G) of NO monomers adsorbed on active sites of sulfated zirconia has been observed. The sites where this form is stabilized seem to be very strong electron accepters responsible for the formation of radical cations after adsorption of aromatic molecules,