Forces allied with the country's unity government discovered the archives as they searched buildings seized during their months-long battle to oust IS from its coastal bastion of Sirte.
The documents belonged to the jihadists' "Judicial and Complaints Department".
Published on pro-government forces' Facebook pages, they include marriage contracts and divorce rulings without any real name or personal information.
In one example from November 31, 2015, Abu Mansour, a Tunisian born in 1977, married a Nigerian called Miriam, in the presence of Sudanese and Malian witnesses.
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Fatima, from Nigeria, was promised a Kalashnikov assault rifle in case of divorce or if her husband, Malian Abu Said, died.
IS jihadists took over Sirte in June 2015 and imposed a reign of terror including executions in public squares.
They patrolled the streets in 4x4s to ensure that men were observing prayer times and women were not venturing out alone.
Forces allied with Libya's Government of National Accord launched an offensive on May 12 to retake the port city that was the home town of late dictator Moamer Kadhafi.
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