Borosilicate glass and glass-ceramics are the most common sealant materials for planar solid oxide fuel cells (SOFCs). This study focuses on the fundamentals of deposition and poisoning of volatile boron species from the borosilicate glass on the electrocatalytic activity and microstructure of La0.8Sr0.2MnO3 (LSM) cathodes under accelerated SOFC operation conditions, using EIS, SEM, FIB-SEM, HRTEM, NanoSIMS, XPS and ICP-OES. The presence of boron species poisons and deteriorates the electrochemical activity and stability for the O2 reduction reaction on the LSM cathodes. Boron deposition occurs randomly on the LSM electrode surface under open circuit but is driven to the electrode/electrolyte interface region under cathodic polarization conditions, resulting in the formation of LaBO3 and Mn2O3 and the disintegration of the LSM perovskite structure. The preferential boron deposition at the interface is most likely due to the increased activity of the highly energetic lanthanum at LSM lattice sites at the three phase boundary region under cathodic polarization conditions, accelerating the reaction rate between the gaseous boron species and energetic La. This study provides a fundamental insight into the boron deposition and its interaction with SOFC cathodes.