We have observed the NMR of 57Fe on the a and d sublattices of YIG, GdIG, and LuIG between 4° and 200°K. Using the spin‐echo technique the frequencies could be determined to within a few kHz. For LuIG the determination of the frequency was less accurate due to the presence of the lutetium resonances which overlay the iron resonances. Assuming the frequency is proportional to the sublattice magnetization, the data were fitted to the equation derived from spin‐wave theory. For YIG and LuIG inclusion of the T 7/2 term was necessary to represent the data above 40°K. Using the theory of noninteracting spin waves we have derived expressions for the sublattice magnetizations in terms of the exchange integrals Jaa, Jad, Jdd , and J′ad , where J′ad describes interactions between next‐nearest‐neighboring pairs of spins on the a and d sublattices. Our experimental results indicate that Jaa and Jdd are not negligible. The T3/2 coefficients were found to be about 15% larger for LuIG than for YIG. Our value of the exchange stiffness constant of YIG is 10% larger than those determined calorimetrically and hence agrees with values determined by microwave instability measurements. For LuIG our value of this constant agrees with the specific heat value.