Abstract We report on X-ray reflectivity experiments of thin liquid heptane films. The films were examined using X-ray reflectivity and modelled both as a layered system using the Parratt algorithm and using a ´phase guessing´ inversion method. Our results combined with simulations of films of shrinking thickness show that small temperature instabilities during the investigation can cause artifacts in the electron density profile. These can easily be mistaken for density os cillations near the interface as might be expected for liquid molecules near a hard wall.