Vowel modifications changed one of the vowels in word into a long vowel or diphthong. The change could apply to one of the characteristic-vowels or a variant vowel appearing from subordinate-vowel-variation. There were two types of modification: vowel-lengthening and a-fortification. As noted under root-modifications, at most one vowel in the word was modified in this way (SD/424).

This vowel modification did not change the identity of the characteristic vowel. For example, a word such as ✶gaimal < √GIMIL was possible, with a variant vowel a in the second syllable and an a-fortified characteristic vowel i in the first syllable (SD/434). In particular, subordinate vowel variations were not considered a vowel modification, so that both a variant vowel and a modified vowel could occur in the same word. In fact, the variant vowel could be the vowel that was modified, as in the example ✶kulīb < √KULUB (SD/425), with a lengthened variant i in the second syllable.