Ru-core Pt-shell nanoparticles show an enhanced electrocatalytic activity for the methanol oxidation reaction (MOR) compared to the respective monometallic or most bimetallic alloy systems. Due to their ill-defined structure, however, a simple interpretation of these findings is challenging. In the present work we present results of a systematic study on the influence of the Pt film thickness of structurally well-defined PtX-ML/Ru(0001) model electrodes with varying Pt film thickness (1.1-5.5 monolayers (ML)) on their MOR performance. The electrodes are prepared and structurally characterized by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. The electrochemical/-catalytic properties of the electrodes are evaluated in an electrochemical flow cell, which is attached to the UHV system and combined with a mass spectrometer in a differential electrochemical mass spectrometry (DEMS) set-up, allowing for online detection of volatile reaction products/intermediates. We demonstrate that the MOR activity of PtX-ML/Ru(0001) increases with increasing Pt film thickness and that thicker Pt films (>2.5 ML) are significantly more active than Pt(111) and thinner Pt films, whereas the selectivity for CO2 formation remains essentially constant. We explain this trend in the activity via the Sabatier principle, where the higher activities of the Pt film electrodes with thicker Pt films compared to both Pt(111) and electrodes with thinner Pt films are due to a lower binding strength of adsorbed species than on Pt(111), due to vertical electronic ligand effects (Ru - Pt interactions) and compressive strain effects in the pseudomorphic films, and due to an increasing binding strength with increasing Pt film thickness, due to decreasing electronic ligand effects. In this picture the Pt5.5-ML/Ru(0001) electrode exhibits the best value of the binding strength and thus the highest rates, while for electrodes with lower/higher binding strength the rates are lower. Consequences of these findings for bimetallic PtRu nanoparticle catalysts are discussed. (C) 2019 Elsevier Ltd. All rights reserved.