With interest rates near historic lows, Wells Fargo (WFC), Sovereign (STD), and other banks are offering certificates of deposit tied to equity indexes, commodities, or currencies. The pitch: They're safe—backed by the Federal Deposit Insurance Corp.—and could pay out more than conventional CDs. But they're complicated, and buyers could end up earning little or no interest.

"Market-linked CDs are a great selling tool [to appeal to] nervous Nellies," says Thomas Balcom, founder of Ibis Wealth Management in Boca Raton, Fla. Wells Fargo sold about $5 billion of its market-tied CDs last year. Sovereign's, which bet on the performance of the Standard & Poor's 500-stock index, have attracted $1 billion since October. Yet "there are other ways to get similar returns without all the complications," says Thomas Orecchio, principal at Modera Wealth Management in Westwood, N.J.

Sovereign's market-linked CD comes as a package consisting of a conventional six-month CD paying 2 percent interest and a three-year CD that pays 2.9 percent each year that the S&P 500 is higher than on the purchase date. If the index hasn't gained ground, the owner gets no interest for that year. The minimum investment is $5,000, split evenly between the two CDs.

Safety-minded investors can create something similar to a market-linked CD using a Treasury security known as a zero-coupon bond, which is sold at a discount to the maturity value. For instance, earlier this month a saver with $1,000 could have laid out $910 to buy a $1,000 zero-coupon bond maturing in 2015, and invested the remaining $90 in a stock index fund.

The bottom line: Investors who put money into market-linked certificates of deposit may end up earning little or no interest.

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