The Gorontalo–Mongondow languages are divided into two branches:[1] Similarities between Mongondow and the languages of the Philippines were already recognized in the first half of the 20th century.
[3] Noorduyn (1982) presented phonological and morphological evidence for a close connection between Gorontalo and Mongondow,[4] while the full extent of the subgroup including all other Gorontalic languages was established by Usup (1986).
[1] Blust (1991) has shown that the Gorontalo–Mongondow languages link up with many languages of the central and southern Philippines in the Greater Central Philippine subgroup.
[5] The following table exemplifies the close relationship, listing Greater Central Philippine innovations which are found in Mongondow (representing the Gorontalo–Mongondow languages) and Tagalog (the northernmost member of the Greater Central Philippine subgroup): The lexicon and phonology of Proto-Gorontalo-Mongondow has been reconstructed by Usup (1986).
[6] Initial sound changes from Proto-Greater Central Philippine:[7]