In March 1656, Boym started his return trip to China. Out of eight priests accompanying him, only four survived the journey. Upon reaching Goa it turned out that Yongli's situation was dire and that the local Portuguese administration, despite direct orders from the monarch, did not want to let Boym travel to Macau. This was in order not to compromise their commercial enterprises with the victorious Manchu. Boym again ignored the Portuguese monopoly by travelling on foot, this time by an uncharted route to Ayutthaya, the capital of Siam. He arrived there in early 1658, and hired a ship from pirates, with which he sailed to northern Vietnam. In Hanoi, Boym tried to procure a guide to lead him and the priests travelling with him to Yunnan. However, he was unsuccessful and he had to continue the journey alone, with the assistance only of Chang, who had travelled with him all the way to Europe and back. They reached the Chinese province of Guangxi, but on 22 June 1659 Boym died, having failed to reach the emperor's court. The burial place remains unknown.