The study is dedicated to the exploration of the language employed in Old French official documents from the Luxembourg area. First, it provides an edition of all the original documents in Old French pertaining to the governments of Countess Ermesindes (1226-1247) and Count Henry V (1247-1281) of Luxembourg. In addition, it offers a detailed analysis of the language of these documents and thus represents a contribution to historical linguistics research on scriptae. Accordingly, the methods employed are a combination of historical and linguistic (drawing on the methods used for analysis of text varieties); they are used to investigate the written idioms of 13th century Northeast France. The study contains extensive material on the scriptae of the Middle Ages.