Interest on Excess Reserves (IOER)

What Was Interest on Excess Reserves?

Interest on excess reserves, or IOER, was the interest rate the Federal Reserve paid depository institutions on reserve balances held above required reserves. It became an important monetary-policy tool after the Fed received authority to pay interest on reserves during the 2008 financial crisis.

IOER is now mainly a historical term. The Fed later simplified the reserve-interest framework and uses interest on reserve balances, or IORB, as the current administered rate. Understanding IOER still helps readers understand how the Fed moved from a scarce-reserves system toward an ample-reserves operating framework.

How IOER Worked

Banks hold reserve balances at the Federal Reserve. When reserves were abundant after crisis-era asset purchases, the Fed needed a way to keep short-term market rates from falling too far below its target range. Paying interest on reserves helped create a floor-like incentive. A bank generally would not want to lend reserves in the federal funds market at a meaningfully lower rate than it could earn from the Fed.

That did not create a perfect floor in every market, but it gave the Fed an administered rate that influenced money-market pricing.

Why It Mattered for Monetary Policy

Before the global financial crisis, the Fed mainly steered the federal funds rate by managing the quantity of reserves. After reserves became abundant, controlling the price paid on reserves became more important. IOER helped the Fed raise or lower short-term rates without needing to drain reserves back to a scarce level.

This changed how market participants read policy. The target range for the federal funds rate remained central, but administered rates such as IOER, the overnight reverse repo rate, and later IORB became key implementation tools.

IOER Versus IORB

The shift to IORB matters because reserve requirements were reduced to zero in 2020, making the old required-versus-excess distinction less useful in practice.

Bank and Market Effects

For banks, IOER affected the return on a risk-free balance held at the Fed. That could influence incentives to hold reserves, lend in money markets, or adjust balance-sheet usage. For markets, changes in IOER could move short-term rates, Treasury bill yields, repo pricing, and expectations about the stance of monetary policy.

It is a technical term, but the financial consequence is broad: the rate helped transmit Fed policy into the short end of the yield curve.

Why the Old Name Still Appears

Readers may still encounter IOER in older Fed statements, academic papers, bank analysis, and commentary about the post-2008 operating framework. In those contexts, the term usually refers to the administered rate that helped anchor short-term money-market rates when reserves were abundant.

When reading current Fed material, however, IORB is the cleaner term. Treating IOER and IORB as identical in every context can create confusion because the required-versus-excess distinction belonged to an older reserve-requirement setup. The name is therefore useful for history, but less useful for describing today's operating framework.

The Bottom Line

Interest on excess reserves was a Federal Reserve administered rate paid on bank reserves above required levels. It became important in the post-2008 ample-reserves framework and is best understood today as the predecessor to the simpler IORB system used to help control short-term interest rates.