There are a few attested genitive inflections of Ilkorin words in The Etymologies, most of them as elements in compounds. The singular genitive inflections end in the suffix -a and the plural inflections in -ion. Based on the example Dor. naugol → naugla (Ety/NAUK), it seems the genitive suffix could cause vowel loss (syncope) just as with plural nouns. Based on the example Ilk. thorn → thurnion (Ety/THOR), the genitive plural could also cause i-affection in the Ilkorin dialect like plural nouns as well, though not in the Doriathrin dialect: Dor. than → thanion (LR/405).

The genitive inflection itself is attested as -a(n) (Ety/NAUK). According to that entry, the genitive generally preceded the modified noun in the Doriathrin dialect. This rule may have only applied to that dialect, since there are several counter-examples in general Ilkorin: Ilk. Tor Tinduma and Ilk. Torthurnion. The form -a(n) hints that the genitive might sometimes be expressed with the suffix -n, perhaps after nouns ending in a vowel. If so, this would make the Ilkorin genitive inflection almost the same as the earlier Gnomish genitive, which used -a/-ion after nouns ending in consonants and -n/-thon after nouns ending in vowels (GG/10).