Get ready for a new kind of ad that may appear when you search with Google.

The company is inviting advertisers to help test new "product ads" that display photos and prices of products for sale, including specific offers based on your search. It's a new program that looks like a response to Microsoft and a challenge to Amazon.com.

These new ads could change the look of Google search results, if you're using the site to look for information about products. The testing is going to begin soon in just a few, unspecified areas of the U.S.

But the program is apparently designed not to change the consumer experience as much as give companies using Google's affiliate program more options to sell their wares.

It's probably been in the works for a while, but the timing is interesting, coming as Microsoft repositions its Web search service as a better tool for shopping. Microsoft's Bing search service includes a number of product shopping features for consumers and vendors, returning product images, prices and specific offers when you search for, say, an Apple iPod.

Google product ads may also increase tension with Amazon.com, by giving companies a new alternative to Amazon's affiliate product marketing program and a product search ad program the retailer started last year. (I wonder how commissions will be divvied up on Amazon product ads placed by Google?) It could also be a significant challenge to smaller, product-oriented ad networks such as Seattle's WidgetBucks.

Google Product Ads will appear as sponsored links, separate from the Google's AdWords text ads.

A company spokeswoman initially said the "experiment" began a few weeks ago, then clarified that it has not yet begun, and provided the following statement:

"At Google, we're constantly experimenting with new features, tools and visual representations to improve the user experience and usefulness of our ads. In accordance with that philosophy, we're planning a beta test to show richer product information in the ads for shopping-related queries. This test will only be visible to a small number of U.S. users."

Microsoft and Amazon declined to comment.

An invitation to the beta includes FAQs with this explanation of how product ads differ from Google's AdWords:

"Product ads represent individual offers with product information directly in the ad itself. Unlike keyword targeted text ads, product ads appear when the user's query matches information provided in your product feed. During the beta, product ads are shown independently from text ads."

Google is encouraging testers to be aggressive with the commissions they pay to sites that display their product ads. Its pricing advice:

You specify the commission rate for conversions that take place via clicks on Google product ads. Minimum pricing is your standard publisher rate, plus the network fee. To maximize your competitiveness among advertisers participating in product ads, we recommend a higher commission with the Google product ads relationship.

Google's affiliate program is based on the DoubleClick Performics Affiliate it acquired in March 2008.