The focal point of Ivšić's research was Croatian Štokavian subdialects, on which he published several very important studies (Šaptinovačko narječje, 1907; Današnji posavski govor, 1913).

In the paper Jezik Hrvata kajkavaca (1936), he partitioned dialects of Kajkavian language (not including the goranski subdialects ) into four major groups.

He extensively studied Croatian Glagolitic heritage, especially its special language features,[clarification needed] on which he published several works.

In 1934, he tracked the Baška tablet from Krk to Zagreb and wrote an enthusiastic article about it called Sveta Lucija u Jurandvoru i njezin dragi kamen (Jutarnji list, 1934).

[2] In the spring of 1945, due to alleged collaboration with the enemy, he was sentenced to exile from Zagreb and lost his membership in the Academy of Sciences and Arts.