Conclusions on the lubrication conditions in the sealing gap of rod seals can be drawn by measuring the oil film thickness on the rod after outstroke, for which ellipsometry is used in this paper. A commercially available U-cup rod seal made of polyurethane and a polished, hard-chrome-plated hydraulic rod were used. The film thickness was measured at various rod speeds (5–800 mms^(−1)), operating pressures (up to 25 MPa) and oil viscosities (ISO VG 15 - 460). Discrepancies between the inverse theory of hydrodynamic lubrication (IHL) and empirically measured film thickness are discussed in this paper. An approximation equation to calculate the film thickness of the analyzed sealing system based on the presented measurement data is provided.