The photochemical reduction of xanthone (XO; X = C13H8O) by xanthene (XH(2)) in a sodium dodecyl sulfate micelle was controlled by a resonant microwave pulse in a magnetic field. The product yields were measured by high performance liquid chromatography. At a magnetic field of about 335 mT, a microwave pulse (9380 MHz; 1.0 kW) applied immediately after the laser pulse for 6.0 mu s decreased the escape products, XOH-XOH and XH-XH, to less than one-third of those without the microwave pulse. A microwave pulse much later than the laser pulse or off-resonant to the radical pair had no effect on the product yield. On the other hand, a reverse effect of the microwave pulse was observed on the yield of the cage product, XOH-XH. The yields as a function of the magnetic field correspond to the ESR spectrum of the intermediate radical pair.