The characterisation of nanoparticle structures is the first step towards understanding and optimising their utility in important technological applications such as catalysis. Using newly developed in-situ transmission electron microscopy (TEM) specimen holders, the temperature dependent atomic structure of gold nanoparticles in the size range 5¿12 nm has been investigated. In this size interval, the decahedral morphology has been identified as the most favourable structure at or above room temperature, while particle surface roughening becomes evident above 600°C. An icosahedral transition has also been identified at low temperature in particles under 9 nm in diameter. These experimental results are consistent with recently published temperature dependent equilibrium phase maps for gold nanoparticles. PACS61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots) 64.70.K- Solid¿solid transitions 81.16.-c Methods of nanofabrication and processing 68.35.B- Structure of clean surfaces (and surface reconstruction) 68.37.Lp Transmission electron microscopy (TEM) SubjectsSurfaces, interfaces and thin films Condensed matter: structural, mechanical & thermal@en