The group strove to develop linguistics in a scientific manner; Leskien formulated their main doctrine, namely that phonetic laws do not have exceptions (Ausnahmslosigkeit der Lautgesetze).

Leskien's hypothesis was that phonetic shifts do not occur randomly or haphazardly, but instead are the product of directly observable conditions.

According to this principle long vowels, along with the diphthongs ie and uo, with an acute intonation are shortened in the final syllable of a word.

Leskien is also the author of Handbuch der altbulgarischen Sprache, a guide to the language Old Church Slavonic (3rd ed.

With Karl Brugmann, he edited Litauische Volkslieder und Märchen (“Lithuanian Folk Songs and Tales”; 1882).