European officials are investigating whether Apple and Starbucks are dodging taxes under arrangements with the Irish and Dutch governments.
A unit of automaker Fiat(FIADF), and its tax affairs in Luxembourg, are also covered by the in-depth probe announced by the European Commission on Wednesday.
"In the current context of tight public budgets, it is particularly important that large multinationals pay their fair share of taxes," said European antitrust chief Joaquin Almunia.
Based on a preliminary analysis, the European Commission is concerned that transfer pricing arrangements in each case could underestimate taxable profits, potentially giving an unfair and illegal advantage to the companies.
Transfer pricing allows companies to move taxable profits from one part of the group to another, often in a different country, by charging for goods or services provided internally.
Apple, for example, has paid as little as 2% on profits attributed to its subsidiaries in Ireland, where the top rate of corporate tax is 12.5%.
Tax experts say Apple and others appear to let offshore subsidiaries enjoy outsized profits, allowing them to exploit tax loopholes.
The companies insist they pay all the tax they owe.
"Since the iPhone launched in 2007, our taxes in Ireland have increased tenfold," said an Apple spokesperson. "Success and growth come from the hard work of our Irish employees, not from any special tax deal with the Irish government. We have received no selective treatment from Irish officials."
A Starbucks spokesperson said the company complied with all relevant tax rules and international guidelines. The probe covers the tax affairs of Starbucks in the Netherlands, where the company has a coffee roasting operation and its European headquarters.