This study explores the tone systems of two languages spoken in the northeast part of India: Dimasa and Rabha. This study involves acoustic analysis of data from the two languages collected from extensive fieldwork. The focus of this study is to determine the lexical tonal inventory of Dimasa and Rabha and the assignment of tones in various morphological domains.

In the available literature on Dimasa and Rabha, there are multitudes of conflicting views about their tone systems and its functions. This study resolves these views and confirms that Dimasa and Rabha have three tones each in their tonal inventory namely, rising, mid-level and falling tones that can be assigned to any lexical word. It also confirms that only one tone can be assigned to each underived lexical word regardless of its syllable size. It is also concluded that in case of derived suffixed words, Dimasa retains the tone of both the root and the suffix whereas Rabha retains only the tone of the suffix assigning a default mid tone to the root. This study also provides an optimality theoretical (OT) account of the tonal phenomena in Dimasa and Rabha.