The second‐harmonic, phase‐matching temperature of ``stoichiometric'' LiNbO3crystals, for fundamental radiation at 1.06 &mgr;, has been found to vary by as much as 75°C from crystal to crystal and 10°C within an individual crystal. Measurements on crystals grown in a controlled manner from nonstoichiometric melts have shown that the phase‐matching temperature,T, is an approximately linear function of melt composition,x, obeying the equationT= 50.5 + 32 × 102(x− 0.50). Crystals with phase‐matching temperatures of as high as 186°C have been grown from melts containing excess lithia. Such a crystal has been operated as a second harmonic generator without damage, whereas a comparable radiation produced by a crystal with a low phase‐matching temperature damaged the crystal. A mechanism involving lithium ion vacancies is proposed to explain the results.