In 1877 he graduated as doctor of philosophy with the dissertation: "Widukind, der Sachsenführer nach Geschichte und Sage" (Münster, 1877).
For some time he taught in the public schools of Münster, Arnsberg, and Aachen, developing in the meantime his historical training.
Previously, however, he spent a year at Vienna for improvement in diplomatics at the "Institut fur oesterreichische Geschichtsforschung" under the direction of Theodor von Sickel.
In the autumn of this year he went to Rome, chiefly to collect in the Vatican archives the material for the large works he had in mind.
Diekamp also published between 1878 and 1885 several important studies in different reviews concerning the history of the Middle Ages and diplomatics or official style of the medieval papal documents.