Polydimethylsiloxane-based self-healing is achieved through the tin-catalyzed polycondensation of phase-separated droplets. The catalyst is released into the crack plane when microcapsules containing a tin compound are ruptured by the advancing crack (see figure). This system greatly extends the capability of self-healing polymers by introducing a new, stable healing chemistry into a structural polymer matrix and may provide a route to self-healing in harsh and corrosive environments.