  The transformation of a system from one state to another is often mediated by
a bottleneck in the system's phase space. In chemistry these bottlenecks are
known as \emph{transition states} through which the system has to pass in order
to evolve from reactants to products. The chemical reactions are usually
associated with configurational changes where the reactants and products states
correspond, e.g., to two different isomers or the undissociated and dissociated
state of a molecule or cluster. In this letter we report on a new type of
bottleneck which mediates \emph{kinetic} rather than configurational changes.
The phase space structures associated with such \emph{kinetic transition
states} and their dynamical implications are discussed for the rotational
vibrational motion of a triatomic molecule. An outline of more general related
phase space structures with important dynamical implications is given.
