Siouan languages
In the 19th century, Robert Latham suggested that the Siouan languages are related to the Caddoan and Iroquoian languages. In 1931, Louis Allen presented the first list of systematic correspondences between a set of 25 lexical items in Siouan and Iroquoian. In the 1960s and 1970s, Wallace Chafe further explored the link between Siouan and Caddoan languages. In the 1990s, Marianne Mithun compared the morphology and syntax of all the three families. At present, this Macro-Siouan hypothesis is not considered proved, and the similarities between the three families may instead be due to their protolanguages having been part of a sprachbund.
Pedee people
Little is known about what language the Pedee spoke. Based on a theory proposed by James Mooney in his 1894 "Siouan Tribes of the East", and reinforced by John R. Swanton in his 1936 essay "Early History of the Eastern Siouan Tribes", the Pedee are believed to have spoken a Siouan language, specifically an "Eastern" or "Southeastern" Siouan language. No vocabulary of the Pedee language has been recorded.