The second‐harmonic, phase‐matching temperature of ``stoichiometric'' LiNbO3 crystals, for fundamental radiation at 1.06 μ, 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 equation T = 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.