The three nonlinear coefficientsd31,d22, andd33that describe optical second‐harmonic generation with LiNbO3have been determined for crystals pulled from melts with Li/Nb atom ratios, (Li/Nb)m, of 0. 852, 0. 946, and 1. 083. The coefficientsd22andd33were found to be insensitive to the melt composition whereasd31increased by about 50&percent; as (Li/Nb)mwas increased from 0. 852 to 1. 083. These results suggest a marked Curie temperature dependence of the difference between the two Nb&sngbnd;O bond nonlinearities that give rise to thed's for LiNbO3.