The Illinois spoke a language in the Algonquian language family called "Miami-Illinois." Miami and Illinois peoples could easily understand one another. The only differences in their speech were different ways of pronouncing certain words. For example, the Illinois spoke with short accents while the Miami spoke with long accents. The Miami-Illinois language changed over time. In the 1600s, speakers of the language pronounced the sound of the letter r. However, during the 1700s speakers began to replace the r sound with the sound of the letter l.

Fathers Jacques Gravier and Jean Le Boullenger, two French missionaries who lived among the Illinois in the late 1600s and early 1700s, prepared dictionaries of the Illinois dialect. The dictionaries include Illinois words for many different entities and actions. For example, there are words for the Illinois, people, places, natural resources, and technology. These word lists reflect the rich linguistic heritage of the Illinois people. Until recently, however, there were no living speakers of the Miami-Illinois language and it was considered extinct. Today, descendants of the Miami and Illinois peoples are attempting to reestablish Miami-Illinois as an actively spoken language through a project called the Miami Tribe Language Revitalization Effort. For more information on the Miami Tribe Language Revitalization Effort, contact the Miami Tribe Web site [https://www.geocities.com/RainForest/7156/index.html]