In this paper we report a distinct ion-exchange route for synthesizing nanocrystalline monothiooxides of rare earths (10 nm) using a high-temperature colloid technique in which the conditions required are much milder than those of conventional routes. The Ln(2)O(2)S samples were prepared by direct conversion from rare earth oxide in sulfur ethylenediamine solution that was maintianed at 150 degrees C for about 8 h. r are earth oxide + S2- 150 degrees C, 8 h -->ethylenediamine Ln(2)O(2)S (Ln = Pr, Eu, Gd, etc.).