This is the accepted manuscript. The final version is available at http://pubs.rsc.org/en/content/articlelanding/2015/ra/c5ra09808d#!divAbstract. ; Structural and analytical characterization related to the formation mechanism of Au-Pt heterodimers from polyhedral Pt nanocrystals is reported. The observation of specific lattice strain effects and the emergence of misfit dislocations point to the relevance of the Stranski-Krastanov growth mode as a means of explaining the previously reported dimerisation reaction between Au and Pt. Two sizedependent strain relief mechanisms were identified. For dimers grown from 4.7 nm seeds, the mechanism is related to bulk lattice strain accumulation at {111} planes along with lattice relaxation effects on other crystalline planes. However, for dimers grown from 11.2 nm seed sizes, the formation of misfit dislocations proved to be a highly efficient mechanism by which to release interface mismatch strain. Nanoscale chemical mapping at Au-Pt interfaces also revealed Au-Pt alloying to be unlikely under the mild temperature conditions employed in this work for Au-Pt heterodimer synthesis. ; This work has received funding from the European Union within the 7th Framework Programme under Project Al-NanoFunc CTREGPOT- 2011-1-285895 and Grant Agreement no.312483 (ESTEEM2). We acknowledge financial support from the CSIC (PIE 201460E018), the Spanish Ministry MINECO (project CTQ2012- 32519), Junta de Andaluc?a (FEDER PE2009-FQM-4554, TEP217 and PE2012-TEP-862) and the Graz Centre for Electron Microscopy (ZFE Graz). Thanks go also to the UK Engineering and Physical Sciences Research Council for a studentship for B. R. Knappett (EP/J500380/1).