During the May Coup d'État Wacław Kostek-Biernacki remained neutral, supporting neither Piłsudski nor the government. The press and the National Democrats opposing Piłsudski speculated, that Kostek-Biernacki might have been involved in the disappearance of Gen. Włodzimierz Ostoja-Zagórski, which however was never proven. In any case, around that time Wacław Kostek-Biernacki was made the military area commander of Brześć Litewski, first informally and then since 1930- officially. While neutral during the coup d'état, he was strongly supportive of the Sanacja movement and his former commander Józef Piłsudski himself. The new authorities of Poland did not forget about his merits either and in 1930 he was promoted to the rank of pułkownik (Colonel). During that period he also authored numerous memoirs and monographs on the activities of the PPS and its Combat Organisation, some of which remained best-sellers for most of the 1920s and 1930s.