This article, one of the papers featuring "Semantic and syntactic analyses of proprietive affixes in the languages of North-Eastern Eurasia", aims to provide an overview of the proprietive suffix -TAJ in Khalkha Mongolian, and then to examine some aspects of the suffix. The main topics to be discussed in the latter part of the article are as follows: Firstly, it is claimed that the proprietive suffix, which has been classified as a derivational suffix, shares some characteristics with inflectional suffixes. Secondly, we explore the relationship between the proprietive and the comitative suffixes: one topic that has long been under discussion in Mongolian studies because the discrimination (or the identification) of the two suffixes is not easy due to their identical phonological shape. Thirdly, some semantic characteristics of the derivatives formed from attaching -TAJ are pointed out, focusing in particular on the semantics of the base. Finally, a possible analysis of sentences is presented where derivatives using -TAJ such as xereg-tej "it is necessary to do …" and jos-toj "ought to …" appear in the final position.