Mr Slabbert was best known for his efforts in the late 1980s to open up dialogue between Afrikaners and the then-exiled ANC.
He was one of the few members of South Africa's white-dominated parliament to oppose apartheid.
The ANC said he had made an "indelible mark" in fighting white minority rule.
Mr Slabbert was apparently only persuaded to stand for office after a hard night's drinking.
But having been elected in 1974, he became leader of the Progressive Federal Party.
In 1985, he travelled to Zambia for talks with the still-banned ANC in an unsuccessful bid to get the government to negotiate with all political groups.
The following year, much to his colleagues' surprise, he quit politics saying he refused to be "in the slipstream of the government's repression and incompetence".
Mr Slabbert then formed the Institute for a Democratic Alternative for South Africa - which aimed to bring resistance groups and influential white figures together.
Much to the government's fury in 1987, he lead a group of 60 influential white South Africans to Senegal where they held talks with an ANC delegation.
BBC News
