What Is Municipals-Over-Bonds Spread (MOB)?

What Is Municipals-Over-Bonds Spread (MOB)?

3 Min.


The Municipal Bond Yield Differential, often denoted as MBYD, quantifies the yield variance between municipal bonds and equivalent-maturity Treasury bonds. MBYD serves as a critical gauge for tax planning strategies, its fluctuations primarily tethered to interest rate movements and the tax-free standing of municipal and Treasury bonds.

Deciphering MOB: Unraveling Municipal Bonds vs. Treasury Bonds

MOB, short for Municipals-over-Bonds spread, deciphers the connection between the yields of a municipal bond index and a Treasury bond index.

Municipal denotes the municipal bond index, while Bonds signify the Treasury bond index. The spread quantifies the disparity between these indices. It expands as the divergence between the municipal and Treasury bond indices widens. This widening occurs when municipal bond yields surge at a faster rate than Treasury bond yields, and it narrows when Treasury bond returns outpace those of the municipal index.

Most MOB spread computations typically employ the implied yields from futures prices of municipal and Treasury bonds traded on the Chicago Board of Trade (CBOT). MOB spread predominantly measures the interest rate differential between federal government debt (Treasuries) and state and municipal debt.

Municipal bonds are mainly exempt from federal taxes, in contrast to taxable Treasury bonds. This tax distinction results in varied actual yields, even when both offer the same interest rate. Assuming identical interest rates, municipal bonds yield more to investors due to their tax-exempt status, whereas Treasury bond earnings are subject to taxation.

Interest Rates' Impact on MOB

Interest rates exert a multifaceted influence on the Municipals-over-bonds spread (MOB). The municipals index, comprised of a dynamic array of municipal bonds, experiences periodic adjustments to accommodate new issuances while retiring older bonds. This composition fundamentally governs the index's sensitivity to interest rate fluctuations, with diverse blends of municipal bonds responding differently to rate changes.

In contrast, the Treasury contract tracks the value of a singular 30-year Treasury bond, which remains noncallable. Callable bonds are predominant among municipal bonds. Consequently, when interest rates surge, callable bonds outperform their noncallable counterparts, causing MOB to widen. Conversely, during rate decreases, noncallable bonds surpass callable ones, leading to a narrowing of the spread.

Traders adeptly leverage these distinctions in taxability and callable status within municipal bonds and Treasuries to establish strategic positions.


MBYD is a key tool for tax planning, impacted by interest rates and the tax status of municipal and Treasury bonds. MOB reflects the connection between these bonds, widening with rising interest rates and narrowing when rates fall. Traders use these differences strategically in the municipal and Treasury markets.

Municipals-Over-Bonds Spread (MOB)