Theodor Posewitz (1850–1917), a Hungarian medical doctor and geologist, spent five years in the Dutch East Indies between 1880 and 1885, serving the colonial Dutch army. He spent time on Java, Borneo, Bangka and Billiton islands. He dedicated his spare time to geological exploration. About eighty of his geological studies appeared in geological and geographical journals. His chief work, the first monograph on the geology of Borneo, including geological and mineral resources maps, was published in German in 1889 and in English in 1892. Early years of his studies and the results of his five-year stay in Asia are discussed, based on archival records. The background for the book, Borneo, and its significance for contemporary and modern science is outlined.