This thesis reports absolute rate constants for the ionization of xenon atoms in well defined high Rydberg states by collision with a variety of electron attaching targets. Theoretical models view such collisions as being dominated by the interaction between the Rydberg electron and the attaching molecule, with the xenon ion core playing only a minor role. It is thus predicted that if the ion products of a collision between a Rydberg atom and an attaching molecule are identical to those resulting from a collision of a free electron with the same molecule, then the rate constant for collisional ionization of the high Rydberg atom colliding with an attaching molecule is the same as the rate constant for free electron attachment to the same molecule, providing the free electron has the same energy as the Rydberg electron, A time of flight technique has been employed to identify the charged products of the observed reactions in order to verify the applicability of the theory. The present results are compared with free electron attachment data, and the results in many cases confirm the theory.