The effect of modifying the composition of a glass matrix based on the Ga 2S 3-GeS 2:Pr 3+ system due to the addition of La 2S 3 on the structure and the optical and luminescent properties of these glasses has been studied. It has been shown that the addition of La 2S 3 leads to changes in the nearest structural environment of Ga, Ge, and S and increases the degree of ionicity of the bonds of the Pr 3+ ion. Despite the existence of a large glass formation region in the Ga 2S 3-GeS 2-La 2S 3 system and the structural and chemical similarity of La and Pr, La 2S 3 does not promote a more uniform distribution of Pr 3+ ions in the glass matrix, and thus does not reduce the concentration quenching of the luminescence of Pr 3+ ions. However, the addition of La 2S 3 increases the probability of emission of Pr 3+ ions and decreases the radiative lifetime. Additionally, it was shown that, when studying the structure and luminescent properties of glasses with La, it is necessary to take into account a significant concentration of rare earth traces (Pr and Nd).