The complexation of thorium (IV) ions in aqueous solutions by pyoverdi
ne A has been studied. On the basis of a simple model, the thermodynam
ical equilibrium constant was determined and the value K-Th =(0.15+/-0
.03) M was compared with those previously obtained with uranyl ions. H
owever, it is shown that the set of data obtained does not give defini
te preference to this model with respect to another that involves comp
lexation with 2+ charged thorium ions. (C) 1998 Elsevier Science S.A.