Comparing the results of the censuses of the language of daily use in the Bohemian lands before the First World War, and nationality censuses after the First World War, has been a subject of scientific and journalistic interest for almost a century. Until now, Czech historians have considered the results of Cisleithanian censuses to be imprecise and underestimated the Czech language use, while considering the results of nationality censuses in 1921 and 1930 to be exact. This study proves that such judgments are too simplistic, since even subsequent to 1918 state policy significantly affected census results, and furthermore there was a part of the population which could not clearly be fitted into one particular nationality or one language. The study analyses the factors which contributed to the increase in Czech nationality and decline in German and Polish nationality subsequent to 1918. It compares the weight of various factors on a countrywide scale, and also on the scale of different regions and locations.