Threshold spectra of several isotopomers of ozone are studied using accurate quantum mechanical calculations and an ab initio potential energy surface. Shallow van der Waals minima in the dissociation channels, separated from the deep main wells by an 80 cm(-1) high barrier, are shown to accommodate long progressions of assignable states. As a result, dense vibrational spectrum of ozone near dissociation is dominated by van der Waals-type states for all studied isotope compositions. (C) 2003 American Institute of Physics.