What foreign exchange reserves countries usually hold that they are selling? Are they selling bonds?

Yes, they are absolutely selling bonds. In fact, bonds make up the vast majority of what a country is actually selling when it "uses its foreign exchange reserves."

A common misconception is that a central bank's foreign exchange (FX) reserves are just giant vaults filled with pallets of physical US hundred-dollar bills. In reality, holding physical cash earns zero interest and constantly loses purchasing power to inflation. To avoid this, central banks invest the bulk of their reserves into highly liquid, ultra-safe financial assets—primarily the government debt of other nations.

Here is what central banks actually hold in their FX reserves, and what they are forced to sell during an economic shock:

The Four Pillars of FX Reserves

A standard central bank portfolio is divided into four main categories:

1. Foreign Currency Assets (The absolute majority) This usually makes up 60% to 85% of a nation's total reserves. It is entirely composed of assets denominated in foreign fiat currencies (most heavily the US Dollar, followed by the Euro, Japanese Yen, and British Pound). This category is split into two parts:

• Sovereign Bonds: The lion's share of these assets are short-to-medium-term government bonds of other nations (e.g., US Treasury bills, German Bunds, UK Gilts).
• Bank Deposits: Electronic cash held in accounts at the Bank for International Settlements (BIS) or other foreign central banks (like the US Federal Reserve).

2. Gold Historically the bedrock of reserves, physical gold is held as a hedge against inflation and currency devaluation. However, gold is relatively illiquid—it takes time, complex logistics, and willing institutional buyers to sell massive amounts of physical gold, making it a poor tool for immediate, overnight currency defense.
3. Special Drawing Rights (SDRs) An artificial international reserve asset created by the International Monetary Fund (IMF). SDRs are not a currency themselves, but they represent a claim to currency held by IMF member countries that can be exchanged when needed.
4. Reserve Tranche Position This is the portion of a country's required quota of currency that it must provide to the IMF. A country can access and draw upon this quota at any time without conditions or interest penalties.

The Mechanics of Selling Bonds

When an oil shock hits or a domestic currency starts crashing against the dollar, a central bank needs actual, spendable US Dollars immediately to intervene in the currency markets or to pay international oil suppliers.

Here is how that drawdown works in practice:

• Step 1: Drain the Cash Deposits. The central bank will first use its highly liquid electronic cash deposits held at foreign commercial banks or the BIS.
• Step 2: Liquidate Short-Term Bonds. Once the immediate cash buffer is gone, the central bank begins selling off its holdings of US Treasury bills and other short-term sovereign debt on the open market. Because the US Treasury market is the deepest and most liquid financial market in the world, a central bank can sell billions of dollars worth of bonds in a matter of seconds to raise the cash it needs.
• Step 3: Repurchase Agreements (Repo). Sometimes, instead of selling the bond outright and losing the asset forever, a central bank will use a "repo" transaction. They temporarily trade their US Treasury bonds for cash with another institution (like the US Federal Reserve), agreeing to buy the bonds back later at a slightly higher price once the crisis stabilizes.

So, when headlines say a country "burned through $10 billion in reserves this month," they aren't loading cash onto planes. They are actively selling $10 billion worth of US Treasuries and other government bonds to raise the liquidity needed to keep their economy functioning.