Understanding Municipal Bond Credit Ratings

Every municipal bond you will ever consider comes with one or more credit ratings — usually from Moody's, Standard & Poor's, and occasionally Fitch. These ratings compress a 300-page analysis of an issuer's finances, economy, management and debt structure into a single alphanumeric symbol like Aa2 or AA-. That compression is genuinely useful. It also masks an enormous amount of nuance that experienced muni investors learn to read back into every position they own.

How ratings actually work

The three major nationally recognized statistical rating organizations (NRSROs) use similar but not identical scales. All three run from a top rating (Aaa or AAA) down to speculative-grade categories that begin at Ba1 / BB+. Investment grade is defined as Baa3 / BBB- and above; anything below is considered high yield. What a rating actually measures is the probability that the issuer will make full and timely payment of principal and interest — nothing more. A rating is not an opinion about market price, liquidity, or the wisdom of investing at any given yield.

What goes into the letter

A rating agency analyst evaluating a general obligation credit reviews four broad areas: the local economy (population trends, industry concentration, unemployment, income levels), the issuer's financial position (fund balance, liquidity, revenue diversity), debt and pension liabilities (direct debt per capita, funded ratios, amortization), and management (multi-year forecasting, reserve policies, willingness to raise revenue). Revenue bonds add a fifth area: the specific project or system's cash-flow coverage. The output is not just a rating — it is a written report that runs 10 to 40 pages and is publicly available on EMMA. Read the report, not just the letter.

Default rates by rating

Historical data is the reason muni bonds sit in so many conservative portfolios. Cumulative 10-year default rates on investment-grade municipal bonds are dramatically lower than on comparably rated corporates. Even A-rated munis have posted 10-year default rates near 0.1%, versus close to 2% for A-rated corporates. Ratings are meaningfully lower on the muni scale, which is why an AA-rated muni is often as safe as a AAA-rated corporate.

That rating-scale difference is one of the central reasons munis outperform comparably rated corporates on a default-adjusted basis — a comparison we break down head-to-head in our article on municipal bonds vs corporate bonds.

Split ratings and what to do about them

It is common to see a bond rated Aa2 by Moody's and AA- by S&P — a one-notch split. When that happens, most institutional buyers treat the bond according to the lower rating for portfolio-eligibility purposes but underwrite it on the higher rating's assumptions. Larger splits — two notches or more — deserve a real read of both reports to understand what one agency sees that the other does not. Splits often precede a rating change; the agency with the more aggressive view is frequently right within 12 months.

Watchlists, outlooks and material events

Between rating actions, agencies publish outlooks (positive, stable, negative) and watchlists (upgrade, downgrade, evolving). An issuer placed on negative watch has a roughly 40% probability of downgrade within 90 days. Material events — pension funding shortfalls, litigation, contract disputes — must be reported to EMMA within 10 business days. Set up alerts on every CUSIP you own and read every notice.

• Positive outlook: an upgrade is possible within 12–18 months.
• Stable outlook: rating expected to hold over the next 12–18 months.
• Negative outlook: a downgrade is possible within 12–18 months.
• Watchlist: a rating action is expected within 90 days.

Ratings are a floor, not a ceiling

The most experienced muni investors treat ratings as a starting point, not an ending point. They read the agency reports, but they also read the audited financials, the pension actuarial reports, and the local news. A city with a stable AA rating and an aging industrial economy, unfunded pensions, and shrinking tax base is a different investment from a growing sunbelt city with the same rating. The letter is the shorthand. The financials are the sentence.