The place of Archbishop Leo of Ohrid in the history of Byzantino-papal relations is assured by his letter to Latin church leaders denouncing their use of ‘azymes’, fasting on Saturdays and other errors in ritual. The ‘Libellus’, Pope Leo ix's broadside of 1053 denouncing the steps taken against Latin churches in Constantinople and the heresies of the eastern Church, was addressed to Leo of Ohrid as well as to Michael Cerularius, and it mentions the archbishop's letter. This letter's role in putting ‘azymes’ at the heart of east-west controversy has long been under scholarly review. But Leo's other two letters on the ‘azymes’ have been relatively neglected, while his treatise on the spiritual benefits resulting from earthly trials has not been edited and his authorship has been questioned. Through scrutinising all available data about Leo, Elmar Büttner firmly establishes his responsibility for all four pieces. He provides an editio princeps of the Kephalaia together with German translation, and an assessment of the treatise's place in Byzantine ascetic literature. Also supplied are ‘preliminaries’ for a full critical edition of the letters. These actually comprise the reedited texts together with translations and commentaries. Failure to look at every witness (some being in rather inaccessible codices) scarcely detracts from the scholarly worth of Büttner's presentation. Paying close attention to the theological content of the letters and the Kephalaia, Büttner brings out the characteristics of Leo's thought. They are most apparent in the treatise, whose fifty maxims and meditations draw heavily on the Old and New Testaments, and not from the church Fathers. The treatise was addressed to all brothers in Christ rather than specifically to monks, and ‘missionary zeal, rigorous seriousness and unstinting self-sacrifice’ (p. 153) are its outstanding features. Leo was a theologian of stature, and the emphasis on the azymes in his first two letters reflects the view of many of his eastern contemporaries, that faulty ritual had repercussions for theology. As Büttner indicates, Michael Cerularius chose carefully in encouraging Leo to present the case against Latin errors in this vein. This edition, together with a wide-ranging historical survey of Leo's role in the east-west exchanges of the earlier 1050s, is of capital importance. It seems that Leo of Ohrid and, probably, other senior clergymen were set firm against compromise on rites embodying doctrine, which they deemed essentials. In that case the mutual excommunications of papal legates and patriarch in 1054 were, if less than cataclysmic, rather more than ‘non-events’.