The five crew members worked on the Zhenlingyou 778 oil tanker in central China's Hubei province, the official Xinhua News Agency reported.
They were unloading asphalt onto a dock in the port in Wuhan, the capital of Hubei when they were suffocated by the gas Monday, the Changjiang River Administration of Navigational Affairs said.
Police said they were investigating the deaths.
Duggar sisters unveil Christian dating rules in new book
Astronomers offer more expansive view of universe