Suppose you hold a portfolio consisting of $250K worth of 30 day treasury bills.?

Every 30 days your bills mature, and you reinvest the principal ($250,000) in a new batch of bills. Assume that you live on the investment income from your portfolio and that you want to maintain a constant standard of living. Is your portfolio truly riskless?

The principle is as safe as the bonds, which would be extremely safe, but not totally riskless (though the risk is absolutely rock bottom, the US govt has never defaulted on a loan.)

The real problem you'd encounter with this strategy is inflation--over time if you lived off the interest rather than reinvesting it inflation would reduce the purchasing power of the $250k and the interest off of it. Also there are other investment options that will probably give you a better long term return that government bonds, like stocks.

It is riskless if you believe the US Government will always be there to print money. It is fully backed by the government. There is little if any risk. And actually, T-Bills have a maturity of 90 days.

However, maintaining a contstant standard of living is another thing altogether. Risk refers to how safe your money is - will you get it back? Constant standard of living implies that the return (income to can take from the investment) stays the same. Interest rates on T-bills vary over time so your income would vary, but it would be safe. Currently rates are about 4.9% (you'd earn about $12,250 per year on your principal), but they have varied over time. As recently as the early part of our short century they were half what they are now.

The main risk is losing value of your investment income to inflation.

t-bills are issued in 7 day, 28 day, 91 day and 182 day day terms I think. There might acually be another thrown in there occasionally. You will not live very well on the investment income from those bills. They are currently generation about $1000 on 250k every 28 days. Might have to eat dog food and live in a cold dark tent. Also you will not maintain a constant standard of living. Far from it. The government just loves to manipulate interest rates to serve its purpose. The first economic down turn an you will be living on $250 a month if that much. Printing money is the national government passtime.

Hell no your portfolio is not riskless. The risks are at least 3.
1. Risk of inflation
2. Risk of the government reducing interest rates.
3. Risk that the dollar will become worth less and less and less as it has been prone to do.