The Carthusian Order is known for its conservative attitude towards liturgy and music. This article will explore how this attitude played out in practice when the Carthusians were confronted with the introduction of a major new feast. Since its origins in the late eleventh century, the Order incorporated several new feasts in its calendar. These additions were normally made with a significant delay, and almost always without any new chants created for these feasts. The feast of Corpus Christi provides an interesting case study. Contrary to their habit, the Carthusians were apparently quick to adopt it, and they included most of the chants that were compiled and edited for this feast. In doing this, they took the Cibavit eos Mass and the Sacerdos in aeternum Office, most famously found in a late thirteenth-century libellus (F-Pnm, lat. 1143) as a point of departure. The Mass Propers were largely taken over, but small variations in the melodies raise interesting questions about how they were transmitted. By contrast, the office chants were thoroughly reordered and melodically edited in various ways, giving us a tangible sense of how Carthusians dealt with change.