The condition for the release is that one of their jailed leaders should be allowed to be part of a delegation to greet them, a negotiator said in Bogota on Tuesday.
The National Liberation Army (ELN) said it was willing to hand over the first of the hostages to a commission of United Nations and Roman Catholic Church representatives, plus two ELN leaders, one of them still in jail, the negotiator Monsignor Hector Fabio Henao said.
Henao and fellow priests Hugo Puccini and Dario Echeverry are part of a mediating commission that met on Monday with a jailed ELN leader Francisco Galan at the maximum security prison of Itagui, about 400 km northwest of Bogota.
The commission negotiated by short-wave radio from Galan's cell with the ELN commando holding the hostages, Henao said.
After the meeting, Henao announced the ELN's decision to release the hostages, adding, "This is a gesture for the future of the nation."
The rebels said the release would start next week on one condition: "the presence ... of a delegation composed of members of the (Roman) Catholic Church, the United Nations and Francisco Galan."
"This is a gesture for the future of the nation"
Monsignor Hector Fabio Henao
The rebels said the hostages would be freed in Sierra Nevada, a snow-topped mountain range edging the Caribbean Sea and under constant threat from guerrillas and bandits, 950km north of Bogota.
It was in those mountains that the ELN on 12 September seized eight hostages from among a group of tourists, trekking to the 3500-year-old ruins of Colombia's Lost City.
One of the captives, 19-year-old Briton Matthew Scott, later escaped and was rescued.
Colombian President Alvaro Uribe, who set up the commission, offered jailed leaders of the rebel group their freedom if the hostages were released safely.
ELN leader Felipe Torres, a member of the commission, was freed from jail on 8 October. He is one of the people ELN wants on the delegation for the first hostage handover.
Leftist-nationalist rebel group, the ELN, is the second-largest in Colombia with about 4000 armed fighters.