 This study used Bragg coherent diffraction imaging, BCDI, at the I-07 beamline at diamond light source, UK to investigate the diffusion of iron into individual gold nanocrystals in situ at elevated temperatures and its effect on their stability and synthesis. The results show that alloying of iron with gold occurs at sample temperatures of 300 to 500 degrees celsius and deloing of iron from gold at 600 degrees celsius was observed. The volume of the alloy region in the nanocrystals increased with a dose of iron, but no significant time dependence was observed for the structure following each iron deposition, suggesting that the samples reached equilibrium quickly. The resulting phase distribution within the gold nanocrystals after the iron deposition suggests a contraction due to diffusion of iron. BCDI is a useful technique for studying diffusion in three dimensions and alloying behavior in individual crystalline grains. This article was authored by Anna Katrina Sia Standart, Christopher Michael Lynch, Marianne Monteforte and others.