At that time, Müller devoted himself mainly to the stamps of the Austro-Hungarian monarchy.
The Austrian government gave him the task of organising the International Philatelic Exhibition in Vienna (WIPA 1933), one of the largest philatelic exhibitions in the world before World War II.
In the same year, Müller became the president of the Fédération Internationale de la Presse Philatélique.
After the Anschluß of Austria with the German Reich in 1938, Müller took refuge in the United States.
He changed his name to Edwin Mueller and in the following years, he continued his philatelic studies and acted as an auctioneer for the first time.