"Experts have told us that mobile phones are being eavesdropped on more and more, even when they are switched off," Alexandra Boy, spokeswoman for Essen-based speciality chemicals maker Evonik, told AFP.
"The measure applies mostly when sensitive issues are being discussed, for the most part in research and development," she said, confirming a report in business weekly Wirtschaftswoche.
Biscuit tins have a so-called Farraday cage effect, she said, blocking out electromagnetic radiation and therefore preventing people from hacking into mobile phones, not only for calls but also to get hold of emails.
The firm, with 34,000 employees and sales of 13 billion euros ($18.5 billion), is not alone in wanting to defend itself against what experts warn are increasingly sophisticated methods of industrial espionage.
This month the German government opened a new national centre in Bonn to coordinate efforts not only to protect firms from espionage but also state infrastructure from cyberattacks.