The ICO probe - which is still ongoing - launched in 2013 after the Serious Organised Crime Agency handed the regulator a dossier of corporate clients of criminal private investigators, accused of the illegal trade in confidential personal information.
A jury at Maidstone Crown Court returned 15 guilty verdicts this week in relation to the charges, which also cover the company itself.
Private investigators Daniel Summers and Adam John Spear have been found guilty of four charges of the unlawful obtaining of personal data. Colum Tudball, senior loss adjuster at Woodgate & Clark was found guilty of three charges, and Michael Woodgate, director at Woodgate & Clark was found guilty of two charges.
The company, Woodgate & Clark, was also found guilty of two charges. The defendants will be sentenced in January 2018.
ICO head of enforcement Steve Eckersley said: “This case is a strand of an important ICO investigation that our team has put a great deal of work into and we welcome the verdict.
“The illegal trade in confidential personal information threatens people’s privacy and security and it’s right that the actions of those involved should be punished.”