I'm planning to open a Solo 401k this year for my relatively new Sole Prop business (which also happens to be my full time job). I want to make a large pre-tax contribution as I've had a pretty good year and want to reduce my tax burden as much as possible, but I noticed when looking at Vanguard's small business site that they also offer a Roth Solo 401k, i.e. contributions can be made after tax and won't be subject to taxes in retirement, much like a Roth IRA. Because my income is likely to fluctuate by quite a bit from year to year, that got me wondering: What if I made pre-tax contributions to my Solo 401k during my "good" years (when my marginal tax rate is in the 25-28% bracket, or higher), then converted some or all of those contributions to Roth if I were to have a "bad" year (15% or less). I'd be faced with a bigger tax bill during those "bad" years than I otherwise would be, but I'd profit massively on the spread and it would be a huge win. The question is, am I able to do this? After doing some research, the answer that I've found is yes. And no. Yes, because the American Taxpayer Relief Act of 2012 made it possible for small business owners to make pre-tax-to-Roth conversions in their Solo 401k accounts. No, because that feature is still something that your plan administrator has to offer. And none of the major administrators out there (Vanguard, Fidelity, Schwab, etc.) allow for pre-tax-to-Roth conversions within a Solo 401k. In fact, Vanguard was the only firm I could find that even offered a Roth Solo 401k to begin with. So, are any small business owners out there doing this? If so, are you working with a reputable plan administrator? How has your experience been so far? Any feedback on this would be much appreciated.