His investigations were limited in scope, though rarely in depth, by the time restrictions under which he worked. Like a traditional Chinese gentleman, he himself collected rare books, small objets d'art, scroll paintings, and musical instruments. He also scrutinized his treasures with a scholarship and a connoisseurship that won the respect of leading Oriental collectors. He translated a famous Chinese text by Mi Fu on ink stones, the valued objects on which the calligrapher prepares his ink for writing. He was himself a talented calligrapher, a rare achievement for a Westerner. He played the ancient Chinese lute (ch'in) and wrote two monographs about it based on Chinese sources. Most of his publications in these peaceful and seminal years were issued in Peking and Tokyo and won appreciation from both Asian and European scholars.

The holocaust of the Second World War brought an abrupt end to Van Gulik's first Tokyo sojourn. Evacuated in 1942 with other Allied diplomats, he was sent to Chungking as secretary of the Netherlands mission to China. At this remote post he published in 1944 an edition of a rare Chinese work about the Ch'an master Tung-kao, a Buddhist monk who was loyal to the Ming cause in the days of its defeat. He remained in China until the end of the European war in 1945, then returned to The Hague until 1947. The following two years he spent as Councillor of the Dutch embassy in Washington, but in 1949 he finally returned to Japan for a four-year tour of duty.

In 1940 Van Gulik had run across an anonymous eighteenth-century Chinese detective novel that entranced him. Thereafter the vagaries of war and its aftermath cut him off from many of his sources and deprived him of much of his leisure, but he managed to spend odds and ends of free time in studying Chinese popular literature, especially detective and courtroom stories. He prepared an English translation of a traditional detective tale which he published at Tokyo in a limited edition in 1949 under the title Dee Goong An. This story in three episodes was the first of the publications through which the Western.world learned of the exploits of Judge Dee, one of China 's traditional detective heroes.