Chemically Induced Dynamic Nuclear Polarization (CIDNP) technique is applied to study the photochemical reactions of 1-acetylindole-2,3-dione (AID) with several hydrogen donors (1-13). These reactions take place by direct hydrogen abstraction of triplet excited AID from the corresponding 1-13 to give neutral radical pair intermediates, which afford polarized starting materials by back hydrogen transfer and give polarized coupling products by radical pair recombination.