Defect perovskite oxynitrides containing lithium on the octahedral sites, i.e., Sr(1-x)(Li(x)Ta(1-x))(O(3.5-3x-3y)N(2y)), were produced by the ammonolytic heating of a layered perovskite, Sr2Ta2O7, with Li2CO3. The above phase evolution involved the thermal diffusion of Li(+) into the Sr2Ta2O7 lattice and concurrent 3O(2-) → 2N(3-) substitution. The concentrations of Li, N, and anion vacancies in the product oxynitride could be controlled by adjusting the Li2CO3 to Sr2Ta2O7 ratio in the reactant mixture. This study identified three different compositions, Sr(0.92)Li(0.08)Ta(0.92)O(1.91)N(0.89), Sr(0.83)Li(0.17)Ta(0.83)O(1.88)N(0.74) and Sr(0.73)Li(0.27)Ta(0.73)O(1.82)N(0.58), where an increase in the Li composition accompanied decreases in the a- and c-parameters of the tetragonal cell, along with an increase in the band gap. Neutron diffraction and solid state (7)Li nuclear magnetic resonance spectroscopy of Sr(1-x)(Li(x)Ta(1-x))(O(3.5-3x-3y)N(2y)) showed that Li occupied the octahedral site, and these oxynitrides exhibited enhanced bond covalency compared to similar oxides.