Crisp-maker Walkers and chemical distributor Omnichem have been fined a total of £350,000 after a worker was killed by a cloud of toxic gas.
Driver John Marriott, 59, from Melton Mowbray, Leicestershire, died from chlorine dioxide poisoning while transferring chemicals from his lorry.
He inadvertently mixed up the hoses on steel tanks while delivering to a Beaumont Leys site in July 2006.
Both companies admitted two charges under the Health & Safety at Work Act.
Health and Safety Executive (HSE) inspector Sue Thompson said: "This incident was entirely preventable.
"Basic risk assessments and clear procedures could have avoided Mr Marriott's tragic death but as it was there were a catalogue of serious failings.
"Employees who had tried to help Mr Marriott did not know the type of operation that was being carried out, nor the nature of the gas being released."
After realising his mistake, Mr Marriott started to hose the area down but it was too late and he died in hospital a month later.
The HSE said both chemicals were used in Walkers' starch reclamation unit to turn waste starch into food-grade material used to make snack foods.
At Leicester Crown Court, Walkers admitted breaching two sections of the Health and Safety at Work Act 1974 and was fined £200,000 and ordered to pay costs of £38,971.
Omnichem admitted the same breaches and was fined £150,000 and ordered to pay £29,229 in costs.
In a statement, Walkers said it had been "saddened and shocked" by Mr Marriott's death.
It said: "Walkers Snack Foods Limited is committed to ensuring the health and safety for all our employees, customers and visitors, and prompt remedial steps were taken immediately after this incident to ensure this does not happen on one of our sites ever again."