The microscopic structure of silicon-rich and oxygen-rich SiOx (0 < x < 2) layers are very different. Generally, the Random Mixing Model (RMM) is used to describe the oxygen-rich SiOx layer structure in terms of microdomains of high- and low-oxygen content, respectively. We have studied the dimensions of spatial inhomogeneities in a-SiO2/a-Si multilayer stacks obtained by sputter deposition of Si in an Ar-O-2 mixture. By using stacks of very thin layers, we have fabricated spatially inhomogenous structures as a model for the RMM. All stacks have the same total thickness (256 nm) and the thickness/layer is from 128 nm down to 2 nm. The composition and spatial inhomogeneities in the stacks were investigated by ion beam analyses techniques (Rutherford backscattering spectrometry (RBS) and high resolution RBS) and electron paramagnetic resonance (EPR). Infrared spectroscopy (IR) was used to study the local atomic structure of the samples. The EPR measurements, using different values of the microwave power, revealed two types of uncharged dangling bond defects. Their density amounts to approximately 10(20) cm(-3). We are able to detect spatial inhomogeneities down to 2 nm. This value is a firm upper limit for the spatial extension of domains in an RMM material.