In the evening of 29 September 1913, Diesel boarded the post office steamer Dresden in Antwerp on his way to a meeting of the Consolidated Diesel Manufacturing company in London. He took dinner on board the ship and then retired to his cabin at about 10 p.m., leaving word for him to be called the next morning at 6:15 a.m. He was never seen alive again. Ten days later, the crew of the Dutch boat "Coertsen" came upon the corpse of a man floating in the sea. The body was in such an advanced state of decomposition that they did not bring it aboard. Instead, the crew retrieved personal items (pill case, wallet, pocket knife, eyeglass case) from the clothing of the dead man, and returned the body to the sea. On 13 October these items were identified by Rudolf's son, Eugen Diesel, as belonging to his father.
No one knows for sure how or why Diesel was lost overboard. Grosser (1978) presents a credible case for suicide. There are conspiracy theories that suggest that various people's business interests may have provided motives for homicide. Evidence is limited for all explanations.