In northeastern Thailand or “Isan”, Lao, otherwise known as the northeastern Thai dialect, is used as a lingua franca by people living in the area, in which minority ethic groups speak Tai dialects/languages, such as the Nyo, Phuthai, saek, Kaloeng, Yoi as well as others. When tonal languages belonging to the same language family are spoken in an area with the respective speakers in constant contact, it is worthwhile to investigate to what extent their tonal systems and tonal characteristics have influenced each other. To achieve this, the earliest available literature has been examined to obtain what appear to be prototypical tonal system for the three languages. The specific goal, then, was to determine how well these prototypes have been preserved in a region of high language contact. 

The That Phanom district or Nakhon Phanom province was selected as then field site since three Tai languages, Lao, Nyo, and Phuthai, are spoken there. The concept of Gedney’s tone boxes was adopted but lexically modified for the languages in question. The SIL CECIL programme was used for investigating the tonal systems and tonal characteristics. On the basis of the available literature, the prototypes of Lao, Nyo, and Phuthai tones were tentatively determined. 

This study reveals that the prototypes of Phuthai tones are still retained in the Phuthai language, while those of Lao and Nyo have been changed to a certain extent due to mutual interference. That is, their tones have been distorted from the tonal prototypes. This means that both major and minor languages spoken change; and that at least some of the factors. Furthermore, it is questionable whether the new data from the present research supports a single set of tonal prototypes for Lao. To give definite answers, more research needs to be done.