Adverts for Google products occupied 91% of the top ad slots on the firm's search results pages, in a study done for the Wall Street Journal.
Advertising data firm Semrush analysed 25,000 pages for the study.
It looked at 1,000 results for 25 search terms including laptop, watches, speakers and smoke detectors on Google.
Products sold by Google's parent firm Alphabet dominated the top of the results. Google said it had strict rules for buying its advertising space.
A spokesman said that the firm's marketing policies were "consciously and carefully designed" so as not to intervene with ad pricing.
"All our bids are excluded from the auction when determining the price paid by other advertisers and we have strict rules and processes - set to tougher levels than our customers - to govern the use of our own ads products," he said.
The Wall Street Journal reported that in all 1,000 searches for the term "laptop" in the study, the top result was for Google's Chromebook.
Additionally 98% of searches for "watches" resulted in links to Android smartwatch retailers appearing on top.
Android and Chromebook are both owned by Alphabet.
It also noted that alarm products by another Alphabet firm, Nest, featured highly in searches for smoke detectors.
The paper added that after sharing the results with Google many of the ads disappeared - and a BBC test found that other brands including Apple, Lenovo and Apollo now appear to dominate the top of results pages for these terms.
"We think when we look at the screen that Google is placing its ads above others but advertising doesn't work in that way," said Daniel Knapp, senior analyst at IHS.
"It may be that Google is willing to pay more than others or that it has better targeting data for identifying users. The method of the [Wall Street Journal] experiment may also be flawed."
The advertising auctions are automated and run by algorithms, he added.
"We live in an algorithmic world. How are these algorithms making decisions on our behalf and how is that distorting markets and society in general?" he said.
"This is an example of a much bigger underlying issue."