The son of Ludovicus De Deken and Maria-Theresia Van Dyck, he studied theology at the major seminary of Mechelen, then he made his novitiate in the Scheutist Fathers congregation in 1877.
In 1889 De Deken joined a scientific expedition in Turkestan of Gabriel Bonvalot (accompanied by Prince Henri of Orléans, as botanist and photographer) along with his Chinese servant Bartholomeus.
These were the first modern European explorations penetrating the high Tibetan plateau, away from the roads which had been done before by Father Huc and later Nikolai Przewalski.
This expedition was unprecedented, having traveled six thousand kilometers and recovering botanical collections that were subsequently studied at the National Museum of Natural History in Paris.
[1] De Deken left Belgium again in June 1892, this time to king Leopold's domain, Congo Free State, where Scheutists had been present since 1889.
In Wilrijk there is a monument in honor of De Deken, standing with a knee on top of the back of a praying native Congolese.