Graded index coatings of silicon oxynitride have been deposited using ion assisted deposition (IAD). During the IAD process the coated surface is bombarded with low energy reactive ions such that the chemical properties of the coating can be changed in a controlled fashion. We have shown that the chemical composition of SiNXOY can be varied continuously from silicon nitride to silicon oxide. As a result, the index of refraction of the coatings can be varied between 2.1 and 1.45 by varying the gas mixture in the coating system. The process has been used to deposit graded index antireflection coatings and rugate filters. Several diagnostic techniques were employed to examine the optical, mechanical, and chemical properties of the coatings.