The sell-off in mortgage-backed securities ETFs was even more extreme than September's headline $2.4 billion sector outflow suggested. BlackRock's iShares MBS ETF (MBB) suffered roughly $2.67 billion in redemptions in September, its largest monthly outflow since the fund's 2007 inception.
• iShares MBS ETF stock is testing lower boundaries. What's behind MBB weakness?
The magnitude is notable because it eclipsed the fund's previous periods of heavy selling during the Global Financial Crisis and the COVID-19 market shock, according to Barchart, which highlighted the record flow in a recent X post.
Investors dumped the BlackRock Mortgage Bond ETF last month at the fastest pace in history, surpassing the Global Financial Crisis and Covid, Barchart wrote on Oct. 2, 2026.
MBB's Record Exit
The flow is particularly striking given MBB's size. BlackRock's fund has nearly $35.5 billion in net assets, meaning last month's redemptions represented a substantial portion of the fund's asset base. MBB tracks U.S. agency mortgage-backed securities and has been trading since March 2007.
MBB also declined roughly 3% on a total-return basis in September, underperforming the broader U.S. bond market ETF universe.
5% Treasury Yields Change the Equation
The timing matters. The 10-year Treasury yield climbed above 5.2% in late September, reaching levels not seen since 2007. By Oct. 1, the average U.S. 30-year mortgage rate had jumped to 7.28%, its highest level since late 2023.
That creates a tougher proposition for mortgage-backed security (MBS) investors. Treasuries offer high yields without the prepayment and extension risks embedded in mortgage bonds. When rates rise, homeowners are less likely to refinance, potentially extending the duration of MBS portfolios just as bond prices are coming under pressure.
Not All MBS Money Is Leaving
There is also an important wrinkle behind the record MBB redemption. Bloomberg reported BlackRock's model-allocation team sold more than $1 billion of passive MBB in September while allocating about $560 million to an actively managed BlackRock MBS ETF. iShares Mortgage-Backed Securities Active ETF (MBBA) is one of BlackRock's actively managed BlackRock MBS ETFs. According to ETFDb, MBBA saw $561.87 million of inflows in September.
That suggests the record MBB outflow is not necessarily a wholesale rejection of mortgage credit. Part of the move appears to reflect a shift from passive MBS exposure toward active management as rate volatility and prepayment uncertainty increase.
