Cambodia's war crimes court ruled on Monday that the Khmer Rouge prison chief's rights were violated when he was detained for a decade before trial, a decision that could reduce any eventual sentence.
Duch, who was arrested by the Cambodian military in 1999, had to wait in prison until earlier this year to face crimes against humanity charges for his role in the 1975-79 hardline communist regime.
The chief judge at the UN-backed tribunal, Nil Nonn, said it was "unlawful" that the 66-year-old former maths teacher had spent so long in detention before the case came to court.
"The accused, under international law and the law of the kingdom of Cambodia is entitled to a remedy for the time spent in detention under the authority of the military court and the violation of his rights," Nil Nonn said.
The ruling appeared to be a small victory for Duch, whose lawyers in April argued he had been held illegally and urged the judges to compensate by subtracting time from his final sentence and softening their eventual verdict.
Duch, whose real name is Kaing Guek Eav, took the stand in March and accepted responsibility for supervising the extermination of around 15,000 people who passed through the Khmer Rouge's notorious Tuol Sleng prison.
He faces a maximum sentence of life in jail if convicted by the court, which does not have the power to impose the death penalty.
Wearing a striped short-sleeved shirt, Duch told the court on Monday that he was the only staff member at the prison who spoke with senior Khmer Rouge leaders.
"It was me who reported to the upper echelons but it was not done in writing. It was done verbally," Duch said.
However he has consistently denied prosecutors' claims that he played a central role in the Khmer Rouge's iron-fisted rule and maintains he only tortured two people himself and never personally executed anyone.
Khmer Rouge leader Pol Pot died in 1998, and many believe the tribunal is the last chance to find justice for victims of the regime, which killed up to two million people.
The court was formed in 2006 after nearly a decade of wrangling between the United Nations and the Cambodian government, and is expected next year to begin the trial of four other senior Khmer Rouge leaders also in detention.
But the troubled tribunal also faces accusations of interference at the hands of the Cambodian government and claims that local staff were forced to pay kickbacks for their jobs.