The reaction product of pyrene and arsenic pentafluoride has been characterized by nuclear magnetic resonance and electron spin resonance. The ESR absorption line is strongly exchange narrowed. The temperature and frequency dependence of the 19 FT 1 is indicative of spin diffusion-limited relaxation by paramagnetic centers. The 19 FT 2 measurements above 200 K can be explained by assuming hindered rotation of arsenic fluoride units and transient dimer formation. The low temperature second moment can be rationalized in terms of a residual four-fold rotation of a dimer or higher oligomer.