A relaxation peak has been observed in the internal friction spectrum of 18-carat AuAgCu yellow gold alloys at about 750K for 0.5Hz. It is related to the presence of grain boundaries, since it is absent in the spectrum of single crystals. For the 14-carat yellow gold alloy (Au38%Ag32%Cu30%), a phase decomposition between silver-rich and copper-rich solid solution occurs in the same temperature range. The effects of the phase decomposition on the internal friction and the dynamic modulus are studied by isochronal and isothermal measurements and correlated with the microstructure evolution. Upon cooling, the phase decomposition starts at grain boundaries at about 840K, producing a fine lamellar structure, and the grain boundary peak amplitude strongly decreases. As the phase decomposition progresses at the interior of the grains upon further cooling, the internal friction background increases. It remains very high in heating until solid solution homogenisation, which occurs above 890K. Such an increase of the internal friction background is observed also in the single crystalline alloy and may be attributed to the interfaces between lamellae of the silver and copper-rich phase.