Three contract workers died during maintenance work at a plant owned by the Dutch chemical company DSM, the company said in a statement on its website.
"Investigations into the causes of this accident will be carried out by DSM in full cooperation with the local authorities," DSM said on its website.
The three died from asphyxiation on Saturday while cleaning a tank in the plant at Capua, in southern Italy, according to Italian media.
Italy's President Giorgio Napolitano expressed his "indignation for the reoccurrence of fatal accidents caused by serious neglect."
Another man died in Tuscany on Sunday while working at a recycling plant in Tuscany.
Italy has one of the worst records in Europe regarding workplace safety.
In 2009, however, deaths at the workplace reached a historical low at 1,050, down by almost seven percent from 2008 when 1,120 died, according to INAIL, Italy's national workplace insurance institute.