These developments mirror the well-known changes of Noldorin, often called the i-affection or i-umlaut. In Ilkorin, the rule seems to be that when a short [a] or [o] appeared before another syllable containing [i], these vowels shifted to [e] or [u] respectively. One clear example of this change is ᴹ✶Ndanithārō > Dor. Denithor (LR/188). This rule is also important to the formation of plural nouns in Ilkorin, since the plural suffix was -in; consider lalm → (plural) lelmin (Ety/ÁLAM) and thorn → (plural) thurnin (Ety/THOR).

Like the Noldorin rule, it appears that this sound change did not occur for long vowels: consider tóril “queen” (Ety/TĀ), tôr “king” → (plural) tôrin (Ety/BAL) and thôn “pine” → (genitive plural) thonion (Ety/THŌN). It is unclear whether the Ilkorin i-affection carried through to syllables further forward in the word, as was the case for Noldorin.

Special cases: One peculiar example is Ilk. myrilind “nightingale”, which appears beside the form murilind (Ety/MOR, TIN). The form murilind seems to have been produced by regular i-affection from primitive *✶morilind(e), but development of [y] is difficult to explain. Since the initial element of the primitive form was ᴹ✶mǭri, perhaps the [o] was more fronted, resulting in i-affection into [y] instead of [u]. As further evidence of this, the word was adapted into Noldorin as the front-vowel [œ]: mœrilind > merilin(n) (Ety/TIN, EtyAC/TIN). Since this is the only instance of the vowel [y] appearing in Ilkorin, it is hard to figure out what Tolkien intended here.

An i-affection of [e] to [i] also seems logical when comparing to the sound changes of Noldorin, but there is little evidence of such a change. The only attested example is the (hypothetical) name Dilion from the root ᴹ√GYEL (Ety/GYEL), which may have undergone this change when developing from the (even more speculative) primitive form *Gyelion. However, the change did not occur in the plural forms regin from reg (Ety/ERÉK) or Eldin from Eld (Ety/ELED), so it is probably better to assume Dilion is a special case, perhaps a side effect of the change of initial [gj-] to [d-].