Monophthongization is a sound change by which a diphthong becomes a monophthong, a type of vowel shift. In languages that have undergone monophthongization, digraphs that formerly represented diphthongs now represent monophthongs. The opposite of monophthongization is vowel breaking.
Some notable exceptions to this monophthongization are some rural Lebanese dialects, which preserve the original pronunciations of some of the diphthongs. Other urban Lebanese dialects, such as in Beirut, use the mid vowels /e:/ and /o:/. Another exception is the Sfax dialect of Tunisian Arabic, which is known mostly for keeping the Classical Arabic diphthongs /aj/ and /aw/.
Some English sounds that may be perceived by native speakers as single vowels are in fact diphthongs; an example is the vowel sound in pay, pronounced /'pe?/. However, in some dialects (e.g. Scottish English) /e?/ is a monophthong .
Some dialects of English make monophthongs from former diphthongs. For instance, Southern American English tends to realize the diphthong /a?/ as in eye as a long monophthong . Monophthongization is also one of the most widely used and distinguishing feature of African American Vernacular English.
To begin with, the diphthong may change to a monophthong by the dropping of the second element and slight lengthening of the first element: /a?/->[a:], /a?/->[?:], /e?/->[e:], //->[?:]. The vowels /i:/ and /u:/, whose usual forms are in fact slightly diphthongal (close to [?i], [?u]), may undergo the same change and become [i:], [u:].
Next, the following schwa may become non-syllabic, forming a diphthong with (what is now) the preceding monophthong. In certain cases, this diphthong can itself be monophthongized. Thus the original sequences /a?/+/?/ and /a?/+/?/ can end up as simply [?:] and [a:].
For example, the citation form of the word our is /'a/, but in speech, it is often pronounced as  (two syllables or a diphthong), or as a monophthong [?:]. Similarly, fire /'fa/ can reduce to [fa?] or [fa:].
In Hindi, the pure vowels /?:/ and /?:/ are written with the letters for the diphthongs ai and au in Devanagari and related alphabets. The vowel sequences /a:?/ and /a:?/ exist in Hindi, but are written as ?i and ?u, with long initial vowels.
The so-called early frühneuhochdeutsche Monophthongierung (monophthongization in the earliest stages of New High German) is particularly important in today's Standard German. It changed the diphthongs ie [i?], uo [u?] and üe [y?] to respectively ie [i:], u [u:] and ü [y:]:
before 11th century > nowadays
liebe [i?] > liebe [i:]
guote [u?] > gute [u:]
brüeder [y?] > Brüder [y:]
The digraph "ie" has kept its spelling despite monophthongization.
The New High German monophthongization started in the 11th century in the center of the German-speaking area. Bavarian and Alemannic dialects in the south did not undergo the monophthongization changes and thus these dialects remain in an older language state.
Greek underwent monophthongization at many points during its history. For instance, the diphthongs /ei ou/ monophthongized to /e: o:/ around the 5th century BC, and the diphthong /ai/ monophthongized to /e:/ in the Koine Greek period. For more information, see Ancient Greek phonology § Monophthongization and Koine Greek phonology.
French underwent monophthongization and so the digraph ⟨ai⟩, which formerly represented a diphthong, represents the sound /?/ or /e/ in Modern French. Similarly, the digraph ⟨au⟩ and trigraph ⟨eau⟩ represent the monophthong /o/ due to the same process.