A jury at a second inquest into the deaths of two employees at the Bodycote metal refining plant in Hereford overturned one of the original verdicts.
Site manager Stuart Jordan, aged 50, and trainee engineer Richard Clarkson, 29, died in 2004 when argon gas leaked into a work pit.
When Bodycote pleaded guilty to a health and safety breach at Worcester Crown Court, they were fined £533,000 with £200,000 costs.
The first inquest jury had returned verdicts that they were unlawfully killed as a result of health and safety lapses.
After a High Court appeal, Bodycote secured a second inquest, and the judge ruled that the original summing up had been inadequate.
This Friday at Hereford’s Shire Hall, the jury returned a verdict of unlawful killing on Mr Clarkson and a verdict of accidental death on Mr Jordan.
Summing up what witnesses had said, coroner Geraint Williams told the jury that Mr Clarkson, of Madley, had died because his employer was not enforcing health and safety policy at the time.
His senior Stuart Jordan, of Bishopstone, also suffered hypoxia (lack of oxygen) and both died in hospital.
The two were quickly overcome and emergency service personnel had to don breathing apparatus to recover them.
Mr Williams told their families: “You have my profound sympathies.”
Health and safety guidelines as set out by the company had been ignored, the inquest heard, and alarms had not been triggered. Permit to work paperwork had also not been completed.
Insufficient checking in the management chain had contributed towards the deaths of the two men, Mr Williams told the jury.