NEW YORK CITY—Increases in home prices accelerated in September compared to August, S&P Dow Jones Indices said Tuesday. All three of the S&P/Case-Shiller Home Price Indices recorded slightly larger year-over-year gains than their counterparts the month before, as did 17 of the 20 cities in the 20-City Composite index.

The indices were also up on a month-over-month basis, with the National Home Prices Index posting a seasonally adjusted gain of 0.8%, while the 10-City and 20-City Composites both rose 0.6% from August. Prices were up from the month prior in all but one of the 20 cities. None of the cities saw a month-to-month decline.

San Francisco, Denver and Portland reported the highest Y-O-Y gains among the 20 cities, with price increases of 11.2%, 10.9%, and 10.1%, respectively. Phoenix has seen the longest streak of Y-O-Y increases, reporting a gain of 5.3% in September 2015, its tenth consecutive increase in annual price gains.

"Home prices and housing continue to show strength with home prices rising at more than double the rate of inflation," says David M. Blitzer, managing director and chairman of the index committee at S&P Dow Jones Indices. In fact, he says, "The strength seen in home prices since the bottom in 2012 led some to wonder if we're entering a new bubble."

Although bubbles can only be identified in hindsight, Blitzer says, one useful measure compares the increase in home prices to the change in rents. An S&P Dow Jones chart comparing home prices to Y-O-Y changes in rents as reported in the Consumer Price Index shows greater peaks and valleys in the former over the past 33 years.

"Home prices are far more volatile," says Blitzer. "At the same time, the most recent data do not show a huge spread between the two series."

If pricing in for-sale housing is volatile, then sales volume in the sector at the very least is subject to fluctuations. The National Association of Realtors said Monday that existing-home sales in October were off 3.4% from September to a seasonally adjusted annual rate of 5.36 million. However, on a Y-O-Y basis October sales were up 3.9%.

Steve Blitz, chief economist at ITG, questions whether a lack of inventory was really to blame for the pullback, as NAR and other sources have claimed. "Existing home sales are not suffering from a lack of inventory," Blitz says. "If they were, prices would be accelerating and the percentage of sales going to investors wouldn't be almost half of what it was a few years ago."

Sales, says Blitz, "aren't really slowing as much as they have hit a ceiling. This is a ceiling created by affordability, income and employment growth, and, perhaps most important, the great leveraged hunt for homes is no longer part of the broader culture."