Colombian President Juan Manuel Santos says formal peace talks will begin next month in Ecuador with the country's second-largest rebel group, the ELN.
Mr Santos said the ELN had agreed to hand over an ex-congressman, Odin Sanchez, the rebel group had been holding since April 2016. The government was to pardon two rebels.
Mr Santos last year agreed a peace accord with the main rebels, the Farc.
More than 220,000 people have been killed in decades of civil conflict.
Peace talks with the ELN had been due to begin last March.
Exploratory talks have taken place for years but stalled when the ELN refused to hand over all their hostages.
A joint statement said the formal peace talks would start on 7 February in Quito, the Ecuadorian capital.
On 2 February, the ELN will release Mr Sanchez. On the same date the government will grant a pardon to two ELN members.
The ELN has freed several other hostages in the past few months. Mr Sanchez handed himself to the rebels in April last year in exchange for the release of his brother, Patrocinio.
The former governor of north-western Choco province, Patrocinio Sanchez, was held by the rebels for almost three years and had fallen ill when his brother suggested the swap.
The agreement with the larger Farc, the Revolutionary Armed Forces of Colombia, was sealed despite an earlier deal being rejected in a referendum in October.
While it might be easy to think that Colombia is practically at peace after the peace agreement signed with the Farc in late 2016, that is not the case in the areas where the ELN is strong.
A few weeks ago I visited the town of Saravena in Arauca, a province on the border with Venezuela.
Thee were people selling food and drinks in the main square, as well as locals hanging out, all crowding up one end of the square.
The other end was deserted. There was a police station there which had been targeted by ELN snipers and makeshift bombs.
Two days before I arrived, ELN men had ambushed a police patrol, killing two officers and injuring one.
The roads in rural areas were lined in places with soldiers and armoured vehicles.
Saravena's mayor has to take a security detail of some 50 men when venturing outside the urban area.
This is not the description of a peaceful region. But it could become one, if the negotiations between the government and the ELN prove successful.
Who are the ELN rebels?
- Guerrilla soldiers of the National Liberation (ELN),on patrol in Sarare, 27 February, 2000 in the Department of Arauca
- The guerrilla group was founded in 1964 with the stated aim of fighting Colombia's unequal distribution of land and riches, inspired by the Cuban revolution of 1959
- Over the decades, the group has attacked large landholders and multinational companies, and repeatedly blown up oil pipelines
- To finance itself it has resorted to extortion, kidnappings and drug trafficking
- It has been strongest in rural areas