Chhouk Rin faced six charges, including murder and kidnapping, in connection with the deaths of Briton Mark Slater, Frenchman Jean-Michel Braquet and Australian David Wilson.
But Thong Ol, the judge in the Phnom Penh court, said Chhouk Rin was not guilty because of the amnesty law. The 1994 statute granted an amnesty to Khmer Rouge communist guerrillas who defected to the government side within six months of it taking effect.
As Thong Ol read out his verdict, Chhouk Rin's relatives in the courtroom clapped their hands and wept with joy.
A lawyer for te plaintiffsthe families of the victimssaid they were "disappointed that Chhouk Rin has been allowed off on a technicality."
"It would have been preferable to have had a decision on the evidence rather than relying on a piece of legislation in Cambodian law which has yet to be properly analyzed," said William Woodrow, the lawyer for David Wilson's family.
The three young men were killed at Phnom VourVine Mountainin southern Cambodia after Khmer Rouge fighters ambushed the train on which they were traveling and kidnapped them on July 26, 1994.
Chhouk Rin was commander of the Khmer Rouge soldiers who attacked the train and killed the three tourists and 13 Cambodian passengers. He defected with some 200 soldiers before Phnom Vour fell to government troops several months later, and was afterward made a colonel in the Cambodian army.
One Khmer Rouge commander has already been tried and convicted in the slayings of the young tourists and another remains at large despite a long-standing warrant for his arrest.
Several victims' family members, including Wilson's and Braquet's fathers, attended Chhouk Rin's hearing. At one point during defense testimony, Jean-Claude Braquet shouted angrily at Chhouk Rin's lawyer.
The plaintiffs have the right to appeal to a higher court. Woodrow, the lawyer for Wilson's family, said the plaintiffs had not yet decided whether to do so.
Asked his reaction to the verdict, Chhouk Rin said it showed that Cambodia's courts are fair and not swayed by political pressures.
His judgment is not shared by human right groups: Some are concerned that the government may unduly influence planned trials of Khmer Rouge leaders for crimes against humanity committed when they ruled Cambodia in the late 1970s.
According to United Nations Under-Secretary-General for Legal Affairs Hans Corell, the U.N. and Cambodian government have agreed on terms for a tribunal of former Khmer Rouge members. The Cambodian legislature was to approve the plan for a tribunal run by Cambodia with U.N. participation.