Experimental photoion-photoelectron coincidence (PIPECO) spectra have been obtained at different nozzle stagnation pressures for Ar, Kr, Xe, and CO dimers and trimers in the wavelength regions corresponding to the respective ground states through all states accessible with a photon energy of 20 eV. Ionization energies for all ground states were measured and agree well with previously reported values. The formation of stable dimer ions from fragmentation of larger cluster ions initially produced by photoionization is efficient. For nozzle expansion conditions which minimize the formation of clusters larger than dimers, the intensities of the excited PIPECO bands for all clusters, except Ar[subscript]2[superscript]+ and Ar[subscript]3[superscript]+, are found to be negligible with respect to the ground state PIPECO bands. This indicates that these electronically excited ions are dissociative in temporal ranges less than their respective flight times from the ionization region to the ion detector. These PIPECO measurements also provide evidence that some excited states in all of the cluster ions studied are bound. The spectra observed in these experiments are consistent with the interpretation that a van der Waals cluster ion initially formed at the equilibrium nuclear configuration of the neutral may be viewed as a perturbed monomer;The PIPECO technique has been used successfully to obtain the mass-selected threshold photoelectron spectra of the SO and S[subscript]2O transient molecules formed from a microwave discharge, effusive beam source. The PIPECO spectrum of SO shows resolved vibrational structure for both spin orbit components of the ground electronic state (X[superscript]2[pi][subscript]3/2,1/2) over the entire spectral region studied (v = 0-11). The vibrational states for SO[superscript]+(X[superscript]2[pi][subscript]3/2,1/2 v = 3-11) result from autoionization processes. The adiabatic ionization energy, vibrational spacings, and spin orbit splitting deduced from the analysis of this spectrum agree with previously reported values;Analysis of the PIPECO spectrum of S[subscript]2O provides the vibrational spacings and adiabatic ionization energies for the ground and first two excited states. These values agree with previously reported values.