A transmission electron microscopy study of the order-disorder perovskite Pb(In1/2Nb1/2)O3 (PIN) has shown that its microstructures are dependent on the In: Nb B-site cation distribution. In disordered PIN the dielectric properties are those of a relaxor ferroelectric. Lowering the temperature of disordered PIN by means of a liquid nitrogen-cooled stage has been found to stabilize polar ferroelectric micro-domains, ∼ 20 to 30 nm in size. On increasing the long range order of the B-site cations to give ordered domains > 80 nm in size, an antiferroelectric phase is shown to develop that is isostructural with the antiferroelectric perovskite, PbZrO3. Coexisting antiferroelectric and non-antiferroelectric regions have been observed in partially ordered PIN.