CIDEP spectra from free radicals produced by the photolysis of xanthone (Xn) in 2-propanol were investigated with FT-EPR. The spectra were assigned to the 2-hydroxypropan-2-yl (2HP) and xanthone ketyl (XnH) radicals. In pure 2-propanol and 2-propanol containing 10% H2O, the spectra display low field emission/ high field absorption with net emission (E*/A) type polarization. The observed CIDEP pattern is mainly due to the S-To radical pair mechanism (RPM) with minor contributions from the triplet mechanism (TM) and radical triplet pair mechanism (RPM). Upon addition of HCl, the polarization changes to net absorption. The rise time of the absorptive signals is determined by the response time of the spectrometer (similar to 3 x 10(-8) s). Transient optical absorption measurements show that the triplet state of xanthone ((3)Xn*) is quenched by HCl, and the change in spin polarization produced by HCl addition is attributed to this quenching process. The dependence of the CIDEP pattern and triplet xanthone lifetime on HCl concentration shows that both involve a diffusion-controlled process. The main 3Xn(*) quenching process was found to be nonreactive, but it is proposed that its spin selectivity produces spin polarization in the products of the hydrogen abstraction reaction that runs in parallel with this quenching process.