Microcapsules containing reactive diisocyanate for use in self-healing polymers are successfully fabricated via interfacial polymerization of polyurethane (PU). Isocyanates are potential catalyst-free healing agents for use in humid or wet environments. The preparation of PU prepolymer and microencapsulation of isophorone diisocyanate (IPDI) healing agent are presented. Smooth spherical microcapsules of 40-400 pm in diameter are produced by controlling agitation rate (500-1500 rpm) according to a power law relation (n = -2.24). The PU shell wall thickness varies linearly with capsule diameter, such that the capsules wall thickness to diameter ratio is constant (similar to 0.05). High yields (similar to 70%) of a free-flowing powder of IPDI/PU capsules are produced with a liquid core content of 70 wt % as determined by TGA analysis. The microcapsules are stable with only similar to 10 wt % loss of IPDI detected after 6 months storage under ambient conditions. Direct mechanical compression testing of microcapsules reveals a brittle fracture mode and normalized shell wall strength that varies with capsule diameter in a power law fashion (n = -0.77).