Quantitative metathesis (or transfer of quantity) is a specific form of metathesis or transposition (a sound change) involving quantity or vowel length. By this process, two vowels near each other – one long, one short – switch their lengths, so that the long one becomes short, and the short one becomes long.

In theory, the definition includes both

and

but Ancient Greek, which the term was originally created to describe, displays only the former, since the process is part of long-vowel shortening.

In the Attic and Ionic dialects of Ancient Greek, ēo and ēa often exchange length, becoming eō and eā.

This quantitative metathesis is more accurately described as one form of long-vowel shortening. Usually if quantitative metathesis affects a word, other kinds of shortening do as well, in the forms where quantitative metathesis cannot occur:

In general, the vowels affected by this shortening were separated by the Proto-Indo-European semivocalic versions of u or i, usually deleted in later Greek: w (written ϝ or υ̯ ) or y (written ι̯ ).

The Homeric form of the genitive singular in the masculine first declension sometimes undergoes quantitative metathesis: