Surfaces as well as interfaces between two neighboring materials are often subjected to various diffusion processes. Such diffusion processes like oxidation or migration of atoms of neighboring materials can cause layers having gradually varying mechanical properties -- like density, Young’s modulus, or shear modulus -- perpendicular to the surface or interface. The growing miniaturization of MEMS devices enlarges the relative size of these layers and thus enhances the importance of phenomena occurring at such material or phase interfaces thus demanding a detailed quantification of its mechanical properties. In this investigation particular interest is drawn on the question how the propagation characteristics of bulk acoustic waves are affected by diffusion layers. The reflection and transmission behavior of bulk acoustic waves encountering a continuum having a spatially dependent sound velocity is discussed based on numerical simulations as well as on experimental verifications. In contrast to previous work done in this field in which diffusion effects are generally considered as undesirable phenomena, the deliberate realization of microstructures having well defined gradually varying material properties in one or more dimensions represents a goal of this investigation. For metallic thin film multi layers thermally induced diffusion processes have shown to be an easy and reliable technique for the realization of layered structures having continuously varying mechanical properties within several 10 nanometers. Among the experimental methods suitable for the in-depth profiling of submicron metallic thin films providing resolutions of several nanometers, are short pulse laser acoustic methods, Rutherford Backscattering Spectroscopy (RBS), and Glow Discharge Optical Emission Spectroscopy (GDOES). Short pulse laser acoustic methods and Rutherford Backscattering Spectroscopy (RBS) have the advantage to be nondestructive. The short pulse laser acoustic method is described in detail and RBS measurements are presented for verification purposes. Finally potential engineering applications like micro-machined spectrum analyzers, acoustic isolation layers, and band pass filters, operating at very high frequencies are presented.