Officials have said the lorry was travelling at high speed downhill when the driver lost control on a motorway between the capital Nairobi and the market town of Naivasha late on Saturday.
Ten people were also injured.
Mwachi Pius Masai of the National Disaster Management Unit said: “This is a serious chemical incident.
“Police and other rescuers are still on the scene … clearing debris.”
He also warned the number of dead could rise.
The ground around the scene of the crash was scorched and the wreckages of burnt-out vehicles continued to smoulder.
Witness Peter Kimani said: “The lorry was in high speed and lost control after hitting speed bumps and ended up ramming into oncoming cars.”
Kenya has struggled to reduce the rising number of road accidents as more people acquire vehicles in a growing middle class.
In 2013, the government reintroduced breathalyser tests in a bid to cut the number of accidents caused by drink-drivers.
According to the National Transport and Safety Authority, 1,574 people died in road accidents in the first half of this year, 86 more than in the same period last year.