Nuclei below theZ= 50 magic shell gap with A∼100 show a wide variety of structural phenomena. These include excellent examples of vibrational collectivity at low‐spins, which give way to more rotational‐like excitations with increasing angular momentum. In this paper we present recent results from an experiment performed at Yale to study the yrast evolution of states in98,99Mo and101,102Ru. Although the high‐spin data is consistent with predictions from rotational model theories, we propose a simple presciption to distinguish between vibrational and rotational regimes of angular momentum generation. When applied to the nuclei of interest, a clear picture emerges of how these two mechanisms of collective spin generation compete in this region. © 2003 American Institute of Physics