The 10-City Composite Index of house prices declined 0.5% in September and 1.1% during the third quarter, according to real estate market data provider Altos Research.

The index is a measure of home prices based on summaries of metrics associated with active residential property listings. After bottoming out at $470,017 in January, it gradually increased to $509,030 in July before again declining and was $503,401 in September.

Of the 26 markets Altos Research examines, asking prices increased in only five, including Los Angeles, which experienced a 1.5% increase, the largest of the 26 markets. Phoenix had the largest monthly decrease of 3.7%.

Inventory also declined in 23 of 26 markets. All of the 26 markets except San Francisco had a median days-on-market of 100 or more in September. Miami had the slowest inventory turnover rate at 251 days.

Prices are likely to continue showing modest declines throughout the seasonally weak fall and winter months of 2009, Altos said, adding this year’s downturn would likely have been worse were it not for historically low mortgage rates and the first-time home buyer tax credit.

Write to Austin Kilgore.