Cambridge Corpus Christi College Library MS 153, a late ninth century copy of Martianus Capella's De Nuptiis Philologiae et Mercurii, is one of the few extant manuscripts where instances of written Old Welsh can be found. For this thesis the readings in the manuscript have been revised, thus shedding some more light on the edition already provided by Whitley Stokes in 1873. A detailed description of the first 36 glosses and their Latin context (taken both from the standard edition as from MS 153 specifically, which has its own particular errors and omissions) has been provided. Furthermore, in revising the MS, a number of curious readings have been discovered, at least two of which are Old Welsh words hitherto unknown and unedited. These have also been examined and described in detail and the glosses themselves appear as illustrations for the reader to examine at his own leisure. As appendices, two lists of the total corpus of MC glosses (150 in total) with their exact locations in the MS and in the Latin context have been added, one in order of appearance and the other in alphabetical order. Finally, in order to facilitate further research into the subject of these glosses and of MS 153 in general, an overview of the different scribes of the main text has been provided, with examples of their handwriting, as well as a list of paragraphs of the text of Martianus Capella per folio and per column.