Understanding the phase behavior of immiscible elements in bimetallic nanomaterials is essential for controlling their structure and properties. At the nanoscale, the miscibility of these immiscible elements often deviates from their behavior in bulk materials. Despite its significance, comprehensive and quantitative experimental insights into the dynamics of the immiscible-to-miscible transition, and vice versa, remain limited. In this study, we investigate the nucleation and growth kinetics of silica-embedded AgPt nanoparticles (NPs) across a wide range of annealing temperatures (25 °C to 900 °C) to elucidate temperature-dependent nanoalloy phase transitions and NP size distribution. Our findings reveal that the alloy phase persists up to 400 °C, with a corresponding average NP size of ∼2 nm. Beyond this temperature, phase instability begins to occur. We propose a three-stage process of nucleation and growth: (1) initial AgPt nanoalloy formation during deposition, (2) growth via thermal energy-assisted diffusion up to 400 °C, and (3) Ag atom emission from the nanoalloy above 500 °C, indicating Ag diffusion towards the surface, followed by partial sublimation of Ag atoms at 900 °C. These results provide crucial insights into the thermal limits for the dealloying of NPs, growth kinetics, and phase stability or instability under varying thermal conditions.