New physical evidence to support a ketyl radical-anion mechanism for the [3,3]-sigmatropic rearrangement is presented. With use of EPR spectroscopy, spectra are observed that can be attributed to 8, an acyl radical-anion species resulting from a [3,3]-rearrangement; this also functions as a key intermediate in the process. The spectrum of an additional paramagnetic species resulting from further addition of tin-centered radicals to the reaction product was also observed.