I don't think the marginal tax rate has to be high. If the Bloomberg Barclays Agg has a YTW of 2.6% and the Bloomberg Barclays Muni TR index has a YTW of 2.25%, a tax rate of 14% is about the breakeven point. Anything above that effective rate, and this is a rough, back-of-the-envelope calculation, would begin to make munis more attractive. It really depends on what your comparable investments are across the taxable universe when compared to the municipal bond market. I've read that the effective tax rate might be closer to 25%.