This work prepared him for his journey to Tibet in 1844 at the instigation of the vicar apostolic of Mongolia. By September 1844 he reached Dolon Nor and made arrangements for his journey. Soon after, accompanied by his fellow-Vincentian, Joseph Gabet, and a young Mongour priest who had embraced Christianity, he set out. To escape attention the party assumed the dress of lamas or priests. Crossing the Yellow River, they advanced into the terrible sandy tract known as the Ordos Desert. After suffering dreadfully from want of water and fuel they entered Gansu, having recrossed the flooded Yellow River. Upon entering Kuen-Kiang-Hien both missionaries fell dreadfully ill and had to put the journey on hold to rest and recover. By January 1845 they reached Tang-Kiul on the boundary. Rather than take an independent four months journey to Lhasa, they waited eight months for a Tibetan embassy expected to return from Peking. Under an intelligent teacher they meanwhile studied the Tibetan language and Buddhist literature. During three months of their stay they resided in the ancient Kunbum Lamasery, which was said to accommodate 4,000 persons. In late September 1845 they joined the returning embassy, which comprised 2,000 men and 3,700 animals.