Before 1939, Kubiyovych's scholarly works concentrated on the geography and demography of the Carpathian Mountains, especially the eastern Beskids, populated largely by the Ukrainian-speaking minority. At this time, he questioned official Polish statistics concerning the ethnic make-up of the inter-war Polish Republic and maintained that the official numbers on Ukrainians were grossly understated. He was an editor and co-author of the pioneering Ukrainian-language Atlas of Ukraine and Adjacent Lands (1937) and the equally pioneering Ukrainian-language Geography of Ukraine and Neighbouring Lands (1938, 1943).