The article analyzes the relation between Aweti and Kamayurá on different levels. Both languages belong to different branches of the subfamily “Maweti-Guarani” within the large Tupi ‘stock’. Both peoples have arrived rather late to the complex Upper Xinguan society, but probably independently and from different directions. Both resulted from mergers of different groups and suffered a dramatic demographic decline in the first half of last century. There is no concrete evidence that these groups spoke varieties of more than 2 different languages (Pre-Aweti and Pre-Kamayurá). Today, many Aweti are at least passive bilinguals with Kamayurá, their most important allies, but the opposite does not hold. The article also discusses the relations between the languages on the main structural levels. In phonology, the phoneme inventories are compared and the sound changes are listed that occurred from the hypothetical proto-language “Proto-Maweti-Guarani” to Aweti, on the one hand, and to Proto-Tupi-Guarani and further to Kamayurá, on the other. In morpho-syntax, the article offers a comparison of the person systems and of affixes in general, treating in particular the so-called ‘relational prefixes’, which do not exist in Aweti. The most important syntactic shared properties are also listed. There seem to be very little mutual lexical borrowing. In the appendix, a list of more than 60 cognates with reconstructed proto-forms is given. Key-words: Aweti; Kamayurá; Sociolinguistics; History; Phonology.