This site has a whole collection of his online works. There is also a very nice OCRed and formatted PDF version of his La théorie physique, son objet, sa structure (1906).
Paul Needham has just produced the first full-length translation of one of Pierre Duhem's scientific works: Commentary on the Principles of Thermodynamics by Pierre Duhem. From its preface:Pierre Maurice Marie Duhem (1861–1916) held the chair of physics (changed to chair of theoretical physics in 1895) at Bordeaux from 1894 to his death. He established a reputation in both the history and philosophy of science as well as in science (physics and physical chemistry). His pioneering work in medieval science opened up the area as a new discipline in the history of science, and his La théorie physique (Duhem 1906) is a classic in the philosophy of science which is still read and discussed today. Although his work in these two fields is now well represented in English with a number of translations that have appeared in recent decades (Duhem 1892b, 1903, 1902, 1905–1906, 1906, 1908, 1915, 1985, 1996), there is little of his scientific work available in English. The original manuscript of Duhem (1898) was translated by J. E. Trevor, one of the editors of The Journal of Physical Chemistry, for its first issue. But his work almost invariably appeared in French. The present volume contains translations of some of his important early work in thermodynamics, which I hope will contribute to a more balanced picture in English of the breadth of Duhem’s publications and provide a further source of insight into his thought.
Thanks, Alan! I've not had time to check any of this out, perhaps later. I would like to learn French also, but I have no time. Besides, PD's work is too important for it to remain in French.