We have investigated the reactions of ultrathin BaO capping layers with SiO2 and HfO2/SiO2 dielectrics using medium energy ion scattering. BaO readily forms a silicate at high temperatures, intermixing with SiO2. Unlike other silicate-forming systems, Ba diffuses throughout the volume of available SiO2, creating a dilute metal oxide. However, when deposited on a HfO2/SiO2 layer, a Ba silicate layer nucleates at the HfO2/SiO2 interface, leaving an SiO2-like buffer layer. The reaction with SiO2 is markedly different from other silicate-forming metal oxides, where nucleation of distinct phases is observed.