Vapor transport equilibration was used to prepare lithium niobate crystals of a variety of controlled off‐congruent compositions. Crystals were characterized through measurement of the ferroelectric Curie temperatureTcand measurement of the temperature for noncritical phase matchingTPMof second‐harmonic generation from both 1.064‐ and 1.32‐&mgr;m laser sources. Across the majority of the single‐phase region, bothTcandTPMwere observed to vary nearly linearly with Li/Nb ratio. The variation inTPMwith Li/Nb ratio was observed to change direction within the single‐phase region at some small but finite compositional interval away from the Li‐rich phase boundary. Crystals equilibrated to the Li‐rich phase boundary had excellent optical homogeneity. Preparation of Li‐poor crystals was hampered by extremely slow lithium diffusivity and problems with second‐phase precipitation.