The dynamics of radical pairs produced in the photoreduction of several anthraquinone derivatives in SDS micellar solutions were studied by the pulse-mode product-yield-detected ESR (pulse PYESR) technique. In this experiment, spin-adduct yield is usually determined as a function of the delay period for the on/off time of a microwave pulse, which causes a reduction in the spin--adduct yield, from the UV laser. The PYESR response curves, thus obtained, were reproduced by numerical calculation to determine the kinetic parameters. As a result, we found that the water solubility of the semiquinone radical is an important factor which determines the radical-pair lifetime. In addition, we determined the acid-base equilibrium constant for the semiquinone radical of anthraquinone-beta-carboxylic acid entrapped in the SDS micelle to be 6.3 x 10(-7) from the pH dependence of radical-pair lifetime.