Siloxane polymers self-organized into nanostructures with controllable distinct thicknesses from 1 to at least 6 molecular layers, i.e., 1-6 nm, are investigated. Macroscopic manipulation of the surface pressure for these ultrathin films leads to an architecture with a distinct layer thickness and a highly ordered structure as demonstrated by synchrotron X-ray reflectivity measurements performed in situ at the air/water interface. An atactic polymer of cyclolinear methylphenylsiloxane (CL-PMPhSi) that consists of monomer rings formed by six silicon atoms joined by oxygen is investigated in this study. Surface pressure (pi) vs surface area (A) isotherm data for these polymers show up to seven plateaus, and this had been assumed to indicate the formation of multilayers with distinct layer thicknesses consisting of 1, 2, 3, etc. layers. The aim of this work was to prove that such successive multilayers are indeed formed and to study their structure. The high-resolution X-ray reflectivity data (0.01 < q(z) < 0.85 Angstrom(-1)) measured with a liquid surface diffractometer allow a detailed structural analysis using both a model-independent method and slabs as a layer model. The extracted electron density profiles in the direction of the surface normal are in good correspondence with calculated profiles based on the crystallographic structural analysis of the bulk crystalline monomer. The layered structure of the thin film is clearly visible in the electron density profiles.