After more than seven years of an anything-goes market, corporate-bond investors are starting to flex their muscles.

Vodafone Group Plc shelved plans to sell as much as US$2 billion of bonds last week after investors demanded sweetened terms, including provisions that would've protected them against losses in the event the company is taken over, according to people with knowledge of the matter. Such requirements, once rare for investment-grade issues, are being added with increasing frequency as the tide begins to turn after years of easy-money policies that fueled an $11 trillion corporate borrowing binge, data compiled by Bloomberg show.

"I think that deal was a good indicator that investors might be getting a little more serious and saying, 'Hey we're not just going to buy anything'," said Timothy Doubek, a senior portfolio manager at Minneapolis-based Columbia Threadneedle Investments, which manages about $184 billion in fixed-income. "If you want our money, we need covenants to protect us."