In the present article, I analyse the spelling of i‒mutated vowels in Old High German and Old English. In particular, I investigate the question as to why German ‘secondary’ umlaut is represented in writing only at a very late stage while, in Old English, the same mutated vowels are distinguished from non‒umlauted vowels from the earliest sources. I then come to the conclusion that (non‒)representation of i‒mutation depends on a bundle of factors: the phonemic status of mutated vowels, loss or preservation of mutating /i/ or /j/, the frequency of the sounds in question, as well as the availability of graphs to represent i‒mutated vowels. The last factor has to be evaluated with reference to the status of the vernacular as a written language: in Anglo‒Saxon England, writing traditions that are to some extent independent of Latin literacy are quickly established. On the other hand, German vernacular literacy on a larger scale only develops in the 13th ct. – exactly at the time i‒mutation spellings are more consistently employed.