Neutral dissociation of superexcited states of OCS has been studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 14.2-16.8 eV. A two-dimensional spectrum exhibits noticeable features which are assigned as resulting from autoionizing transitions of excited atomic sulfur, S*, from Rydberg states converging to S+(D-2(0)) to S+(S-4(0)). The precursor molecular states leading to S*+CO are considered to be multiple-electron-excited Rydberg states, OCS*(Dis), converging to OCS+ with (2) Sigma(-)- and/or (2) Delta symmetry. The electron signal counts due to autoionization of S* show enhancement at excitation photon energies for the Rydberg states, OCS*(R-B), converging to OCS+((B) over tilde(2) Sigma(+)). These results support a predissociation mechanism for the formation of S-*: conversion from OCS*(R-B) to OCS*(Dis). The quantum yield for the predissociation is evaluated to be similar to 1% at the photon energy corresponding to the 5s sigma state of OCS*(R-B). (C) 1997 American Institute of Physics.