Most ETFs cannot be bought from or sold back to the fund company like regular mutual funds. (The exception: Merrill Lynch's HOLDRs.) Investors can only buy or redeem shares directly from the sponsoring fund company in 50,000-share blocks, and even then, the funds require in-kind transactions. With an in-kind transaction, you don't get cash when you redeem your shares; you get the underlying stocks. In practice, this means that only institutions and the very wealthy can afford to deal directly with the fund companies. The rest of us have to go through a broker to buy and sell shares.

Unlike regular mutual funds, ETFs do not necessarily trade at the net asset values of their underlying holdings. Instead, the market price of an ETF is determined by forces of supply and demand for the ETF shares. To a large extent, the supply and demand for ETF shares are driven by the underlying values of their portfolios, but other factors can and do affect their market prices. As a result, the potential exists for ETFs to trade at prices above or below the value of their underlying portfolios.

However, by permitting large investors to buy or redeem shares in-kind, the fund companies behind ETFs have created a mechanism that should, in theory, help prevent sustained price-to-NAV discrepancies from opening up.

If an ETF traded at a discount to its net asset value, institutional investors could assemble 50,000-share blocks in the open market at the discounted price, redeem them for the underlying stocks, and sell those stocks at a profit. The actual transaction isn't quite that simple, but the idea is the same: The arbitrage opportunity would generate sufficient demand for the discounted ETF shares to close the gap between their market price and the net asset value of the underlying portfolio.

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