Cediwe Mfeti, widow of Phindile Mfeti, a United Democratic Front activist who went missing from the University of Natal in 1987, was sworn in shortly after 9.15am.
The commission was expected to also hear testimony on disappearances, poisoning, ambush and murder.
Among the witnesses due to testify on Tuesday were the widows of murdered UDF activists Fort Calata, Sparrow Umkhonto, Sicelo Mhlawuli.
The mother of Xola Martin Jebe who was allegedly killed in an African National Congress camp outside South Africa or by security forces will also testify before commission chairman Archbishop Desmond Tutu and his fellow commissioners.
The widow of Matthew Goniwe who was also found murdered, Nyameka Goniwe, is due to testify on Wednesday despite reports she would not.
Other witnesses due to give testimony on Tuesday were Singqokwana Malgas who was allegedly tortured in police detention and Robert and Cecilia Cohl who were to testify about their son Bulli Cohl, allegedly killed by police.
Joyce Mthimkhulu was expected to tell the panel about Siphiwo Mthimkhulu who disappeared after allegedly having been tortured and poisoned in detention.
Nonceba Zokwe was to testify that his son Sithmbele Zokwe was allegedly killed by police in the former homeland Transkei.
The last witness due to give testimony on Tuesday was Billy Shiyane who was ambushed and shot by unknown assailants.