This paper reports the internal performance evaluation of S-duct diffusers with different entrance aspect ratios as part of a parametric investigation of a generic S-duct inlet. The generic S-duct diffusers studied had a rectangular entrance (aspect ratios of 1.5 and 2.0) transitioning S-duct diffuser in high-subsonic (Mach number > 0.8) flow. The test section was manufactured using rapid prototyping to facilitate the parametric investigation of the geometry. Streamwise static pressure and exit-plane total pressure were measured in a test-rig using surface pressure taps and a five-probe rotating rake, respectively. The baseline and a variant were simulated through computational fluid dynamics (CFD). The investigation indicated the presence of streamwise and circumferential pressure gradients leading to a three-dimensional flow in the S-duct diffuser and to distortion at the exit plane. The static pressure recovery increased for the diffuser with the higher aspect ratio. Total pressure losses and circumferential and radial distortions at the exit plane were higher than that of the podded nacelle type of inlet. An increase in the total pressure recovery was observed for the increase in the aspect ratio for the baseline area ratio (1.57) S-ducts, but without a clear trend for the other area ratio (1.8) ducts. The work represents the development of a database on the performance of a particular type of generic inlet. This database will be useful for predicting the performance of aero-engines and air vehicles in high-subsonic flight.