The existing grounds for granting and terminating Belarusian and Polish parliamentary mandates from the legal point of view are nowadays rather debatable. The article presents a comparative study of these legal categories based on the considerable historical, regional and legal similarity of the above neighbouring countries and their legal doctrines. The main conclusion of this article is that a large number of similar legislative provisions is related to their essence and is preconditioned mainly by the common origin of Polish and Belarusian parliamentary institutions. Their main point is that the grounds for granting and terminating parliamentary mandates, as an integral element of their legal status, sets the time frame for exercising the authority of both Polish and Belarusian members of parliament. The existing differences between the above legal categories manifest themselves, first and foremost, in their formal expression – the implementation procedure.