While turbine rim sealing flows are an important aspect of turbomachinery design, affecting turbine aerodynamic performance and turbine disk temperatures, the present understanding and predictive capability for such flows is limited. The aim of the present study is to clarify the flow physics involved in rim sealing flows and to provide high-quality experimental data for use in evaluation of computational fluid dynamics (CFD) models. The seal considered is similar to a chute seal previously investigated by other workers, and the study focuses on the inherent unsteadiness of rim seal flows, rather than unsteadiness imposed by the rotating blades. Unsteady pressure measurements from radially and circumferentially distributed transducers are presented for flow in a rotor–stator disk cavity and the rim seal without imposed external flow. The test matrix covered ranges in rotational Reynolds number, , and nondimensional flow rate, , of 2.2–3.0 × 106 and 0–3.5 × 103, respectively. Distinct frequencies are identified in the cavity flow, and detailed analysis of the pressure data associates these with large-scale flow structures rotating about the axis. This confirms the occurrence of such structures as predicted in previously published CFD studies and provides new data for detailed assessment of CFD models.