Magnetic nanoparticles are broadly used for cell tracking in vivo using magnetic resonance imaging. Silica coating, often used for magnetic nanoparticles, represents an inert and stable barrier between the nanoparticle core and cell environment. However, the coating may disintegrate at high temperatures and expose the core. We monitored this process using dynamic relaxometry and confirmed substantial extend of dissolution by transmission electron microscopy. The process probably reflected the equilibrium of solid silica and soluble silicic species in highly dilute suspensions at higher temperature.