The accident took place in Shanxi Province on March 1 when two tankers loaded with methanol - a poisonous and flammable liquid often used as a fuel - ran into each other inside a highway tunnel.
The blast shook the Yanhou Tunnel of the highway linking Shanxi's Jincheng City and Jiyuan City in Henan Province.
Investigators today said leakage from the two crashed methanol tankers killed at least 31 people, with nine still missing, state-run Xinhua news agency reported.
The death toll was updated at a meeting in Jincheng City held by an investigation group sent by China's State Council.