The growth in the uses of lithium niobate for infrared applications has created a need for knowledge of its optical characteristics in the infrared spectral region for the purpose of designing phase-matched or quasi-phase-matched devices. We report measurements of the refractive indices of congruently grown lithium niobate and lithium niobate doped with 5 mol. % magnesium oxide. We use these results to predict the tuning curve of a room-temperature multigrating optical parametric oscillator in each material. (C) 1997 Optical Society of America [S0740-3224(97)00612-7].