The appreciation of a political declaration of 5 November 1916 issued by the Emperor of Germany, William, and the Emperor of Austria, Francis Joseph in which they proclaimed the establishment of a state named the Kingdom of Poland is usually made from a political point of view, especially in the context of internalization of Polish question during the First World War. The purpose of the article is to analyse this declaration as a self-contained state act of supreme authorities of Germany and Austro-Hungary and to examine the legal reasons and circumstances of the announcement of the declaration of 5 November, but also to defi ne the juridical sense and consequences connected with this act, particularly in relation to establishment of Polish state. The constitutive political and legal effect of 5 November Act was the establishment of a state whose institutions, created in 1916–1918, entered into the Polish state established through the self-determination of 1918.