The geometrical arrangement of reagents is an important factor influencing chemical reactions in condensed phases and molecular clusters. In the present study, OH buildup times have been recorded upon photolysis of (N2O)(m)(HI)(n) clusters in order to elucidate the role of the cluster environment on the reaction mechanism. The buildup times were measured for different molecular beam compositions (i.e., degrees of clustering). The buildup time changed from less than or equal to 100 fs at the lowest backing pressures (119 and 132 kPa) to 430 fs at 188 kPa. It is argued that at the lower backing pressures the OH derives primarily from binary N2O-HI complexes. However, regardless of the cluster species involved, the fast OH buildup at the lowest backing pressures suggests a dominant direct oxygen abstraction mechanism rather than reaction via a vibrationally excited intermediate such as HNNOdagger. (C) 1997 American Institute of Physics.