Peter Binder and Franz Radl deny involvement in the December 1993 bombings which injured four people. But they admit to being former members of an extreme right-wing group which had aimed to overthrow the Austrian Republic and restore a Nazi state.
If found guilty, the two men, both 28, could face between 20 years and life imprisonment. But a conviction will be difficult. As the trial opened on Monday, state prosecutors admitted the evidence was circumstantial.
It is already clear that others may have been involved in the bombings. Since Mr Radl and Mr Binder were arrested in December 1993, there have been two more waves of letter bombings and various terrorist actions, the worst of which claimed the lives of four gypsies earlier this year.
An obscure right-wing group that calls itself the "Bavarian Liberation Army" has even claimed responsibility for the bombs, nearly all of which have been aimed at prominent Austrians known for their support for minorities.
The letter-bombs and the racially motivated killings have shocked most Austrians, who are used to seeing their country as a cosy sanctuary from the neo-Nazi violence afflicting neighbouring Germany. They have also prompted criticism of the police, widely seen as being ineffective in its pursuit of the culprits and accused by some of harbouring sympathies for right- wing extremists.
Over the coming month about 130 witnesses are expected to testify in the trial, including Helmut Zilk, the former mayor of Vienna and a champion of the city's Jewish and minority communities who lost three fingers on his left hand when he opened one of 10 letter bombs sent in the first wave in December 1993.
Another key witness is likely to be Gottfried Kussel, the former leader of the now banned Vapo neo-Nazi group to which Mr Binder and Mr Radl both belonged. Kussel personally supervised paramilitary training sessions for his followers, and is serving an 11-year jail sentence for neo-Nazi activity.
In testimony yesterday, Mr Binder, a trained electrical engineer, admitted he had the technical knowledge required to build letter bombs, but denied that he had acted on it. He denied that a stockpile of grenades and TNT explosives discovered at his home shortly before his arrest had been amassed on behalf of Vapo.
In today's hearing, Radl, a law student, is due to answer accusations that he drew up the hit-list for the letter bomb attacks. In addition to Mr Zilk, those targeted included a priest, a television journalist and several human rights activists.