in 1689 gravier was assigned to the illinois in the mississippi valley. first he worked among them at starved rock on the illinois river, where he started compiling a grammar and dictionary. he worked to convert the kaskaskias. in 1694, he helped broker the marriage of the kaskaskia aramepinchieue to the french trader michel aco, which helped to cement the alliance among the jesuits, traders, and kaskaskias. in 1696 gravier was named to found the illinois mission among the illinois, miami, kaskaskia and others of the illiniwek confederacy situated in the mississippi river and illinois river valleys. bishop saint-vallier (la croix), the bishop of quebec, named him vicar general of these missions.