The distributions of Th isotopes in both suspended participate matter and sea water were measured in the western North Pacific and Japan Sea. Suspended particulate matter was collected simultaneously on four 142‐mm‐diameter Nuclepore filters (1.0 μm in pore size) by using an in situ deep‐sea pumping system. Particle associations were found to be 6–40% for228Th, 9–52% for230Th, and 9–72% for232Th with mean values of 26–32% for all three isotopes. In order to explain the observed Th isotope distributions in the Pacific, a reversible exchange model was developed that includes two classes (fine and large) of particle population in the ocean. The fine particles are defined as those collected by filtration and comprising most of the standing crop of particles in seawater with no significant settling flux. The large particles, on the other hand, are defined as those collected via sediment traps and represent a negligible standing stock in the water but play a predominant role in the vertical flux. The model calculation suggests that the transfer of Th from solution to fine particles is controlled by an adsorption/desorption process which depends upon the chemical reactivity of elements. Aggregation of fine particles to fast sinking large particles was estimated to occur throughout the water column on a time scale of one to five months. The large aggregates seem to disintegrate on a time scale of less than a few days, suggesting their extremely