The Missa Mi-mi is a musical setting of the Ordinary of the Mass by Johannes Ockeghem.
It is a motto mass based on one of Ockeghem's own chansons, "Presque transi."
Other early sources are generally untitled, or it is sometimes referred to as "Missa Quarti toni" ( or "Mass in Mode 4").
Earlier scholars generally assumed that 'Mi-mi' referred to the opening bass motive, which would involve the solmization syllables 'mi' and 'mi' in the normal and soft hexachord, respectively.
Rebecca Stewart points out that using the hard hexachord is most likely the way a Renaissance singer would approach this interval, which would avoid the need for mutation.