In its nine-page complaint, filed last week in the San Francisco Superior Court, SurveyMonkey claims that MarketTools has failed to pay several invoices it has issued under two separate agreements between the firms.

The first agreement was for SurveyMonkey to provide ZoomPanel sample to MarketTools. The second relates to a subscription to TrueSample, the survey verification software.

Both products, ZoomPanel and TrueSample, were formally owned by MarketTools, prior to December 2011, when the company was bought by investment firm TPG. While TPG retained the main MarketTools business, ZoomPanel and TrueSample were transferred to SurveyMonkey in an assets-for-equity deal.

According to SurveyMonkey’s complaint, the ZoomPanel and TrueSample agreements were struck with MarketTools following the sale, in January 2012, before being amended and restated in December of the same year.

SurveyMonkey claims that several invoices issued to MarketTools went unpaid during 2012 but were eventually settled in November of that year. However, SurveyMonkey claims a second set of invoices – some dating from June 2013, have so far not been paid in full.

According to source familiar with the lawsuit, the DIY survey software provider is seeking damages of more than $500,000.

SurveyMonkey no longer owns ZoomPanel or TrueSample, having sold the former to Critical Mix and the latter to a group of investors led by Five Peaks Capital Management.

In addition, MarketTools is no longer owned by TPG. It was bought by Dutch online research specialist MetrixLab in July 2012.

A spokesman for MetrixLab declined to comment at this stage, beyond noting that “there are always two sides to a story”. A formal response is expected in due course.