About this time personal considerations induced Giry to devote the greater part of his activity to the study of diplomatic, which had been much neglected at the École des Chartes, but had made great strides in Germany. As assistant (1883) and successor (1885) to Louis de Mas Latrie, Giry restored the study of diplomatic, which had been founded in France by Dom Jean Mabillon, to its legitimate importance. In 1894 he published his _Manuel de diplomatique_, a monument of lucid and well-arranged erudition, which contained the fruits of his long experience of archives, original documents and textual criticism; and his pupils, especially those at the École des Hautes Études, soon caught his enthusiasm. With their collaboration he undertook the preparation of an inventory and, subsequently, of a critical edition of the Carolingian diplomas. By arrangement with E. Mühlbacher and the editors of the _Monumenta Germaniae historica_, this part of the joint work was reserved for Giry. Simultaneously with this work he carried on the publication of the annals of the Carolingian epoch on the model of the German _Jahrbücher_, reserving for himself the reign of Charles the Bald. Of this series his pupils produced in his lifetime _Les Derniers Carolingiens_ (by F. Lot, 1891), _Eudes, comte de Paris et roi de France_ (by E. Favre, 1893), and _Charles le Simple_ (by Eckel, 1899). The biographies of Louis IV. and Hugh Capet and the history of the kingdom of Provence were not published until after his death, and his own unfinished history of Charles the Bald was left to be completed by his pupils. The preliminary work on the Carolingian diplomas involved such lengthy and costly researches that the Académie des Inscriptions et Belles-Lettres took over the expenses after Giry's death.
In the midst of these multifarious labours Giry found time for extensive archaeological researches, and made a special study of the medieval treatises dealing with the technical processes employed in the arts and industries. He prepared a new edition of the monk Theophilus's celebrated treatise, _Diversarum artium schedula_, and for several years devoted his Saturday mornings to laboratory research with the chemist Aimé Girard at the Conservatoire des Arts et Métiers, the results of which were utilized by Marcellin Berthelot in the first volume (1894) of his _Chimie au moyen âge_. Giry took an energetic part in the _Collection de textes relatifs à l'histoire du moyen âge_, which was due in great measure to his initiative. He was appointed director of the section of French history in _La Grande Encyclopédie_, and contributed more than a hundred articles, many of which, e.g. "Archives" and "Diplomatique," were original works. In collaboration with his pupil André Réville, he wrote the chapters on "L'Émancipation des villes, les communes et les bourgeoisies" and "Le Commerce et l'industrie au moyen âge" for the _Histoire générale_ of Lavisse and Rambaud. Giry took a keen interest in politics, joining the republican party and writing numerous articles in the republican newspapers, mainly on historical subjects. He was intensely interested in the Dreyfus case, but his robust constitution was undermined by the anxieties and disappointments occasioned by the Zola trial and the Rennes court-martial, and he died in Paris on the 13th of November 1899.