A key concern for the Dominican Order in the thirteenth century was the uniformity of its liturgy. In the middle of the century, the general chapter sought to consolidate the order’s liturgical practices, issuing a definitive version of the liturgy in 1256. Uniformity between houses within the Dominican Order was vital both for practical reasons and for theological ideals, and therefore a concerted effort was made to ensure that new liturgical books produced after 1256 were carefully copied and checked to ensure that they did not deviate from the official liturgy. Nevertheless, a small number of differences can be found between chants in Dominican manuscripts. What do these tell us about the Dominican perspective on uniformity in chants? What differences were seen as allowable, even normal, while still adhering to the principle that it was unacceptable to make changes to the chants? Six Dominican Mass books will be examined here, all dating from shortly after the reform, in order to examine how the Dominican concern for uniformity was manifest in its chant books.