In a 1969 publication the author proposed a Koiarian family consisting of six languages: Koita, Koiari, Mountain Koiari, Ömie, Managalasi and Barai. This family, part of the putative Trans New Guinea group of Papuan languages, stretches from around Port Moresby on the southern coast of southeast Papua almost to the sea on the north coast at the eastern end of the Hydrographers’ Ranges. In the current work the author enlarges on the lexicostatistically based 1969 work and applies the comparative method of historical linguistics to the Koiarian languages, identifying shared innovations that define subgroups within the family and reconstructing the protophonology and about 120 lexical items of Proto Koiarian. He provides similar reconstructions for Proto Koiaric and Proto Baraic, the languages ancestral to the two major subgroups within Koiarian.