Spiro-orthosilicates, prepared from tetraethyl orthosilicate and a diol (2-butyl-2-ethyl-1,3-propanediol or 2-ethyl-1,3-hexanediol), are used as latent reactive diluents in two-pack polyurethane coatings. A fast curing and long pot life have been achieved because of the latency in the can. Spiroorthosilicates hydrolyse with moisture into a diol and a polysiloxane. Kinetic data on hydrolysis and curing are given. The formation of polysiloxane has been characterised by solid-state29Si NMR. The film properties obtained are excellent. The spiro-orthosilicates-based films are tested as clear coats for car refinishes. In particular, a very good car wash resistance has been obtained, which is combined with other properties, also at a good, competitive level.