Both men travelled to Ukraine from Madrid in July with only €500 ($669) in cash and no return ticket, according to Asturian daily La Nueva España.
After spending five days in Kiev, they travelled to the Crimea and enlisted in the insurgent, pro-Russian Vostok Battalion under the command of Alexander Khodаkovsky, a defector from the Ukrainian security forces and head of the Donetsk People's Republic (DPR) security force.
Muñoz Perez, originally from Madrid, has been resident in Gijón, in Asturias, Spain since 2010 where he was active in the United Left Youth movement. Ángel from Murcia, is a member of the youth wing of Spain's communist party according to Spanish daily El País.
The battalion has incorporated many international volunteers but the two men are the only known Spaniards to enlist.
It is not known if the men have played an active role in the fighting but they have given a number of interviews and posted videos to explain why they are there.
In broken English, they claim that what is seen on Spanish and American TV "isn't the truth".
"There people aren't terrorists, aren't criminals; they are only defending their homes and their families," said Muñoz Pérez in one YouTube clip.
He also asserted the right of the pro-Russian insurgents to have a referendum.
"We are here to defend civil people", said Ángel.
Muñoz Pérez has claimed that he is doing the same as international volunteers did in Spain during its civil war.
"Viva la republica," he added.
The men denied that pro-Russian forces were responsible for downing the Malaysian Airlines Flight MH17 in an incident which killed 295 people.
"It's impossible. There is no gun that can kill [sheet down] a Boeing," said Muñoz Pérez.
The Vostok Battalion has been accused by the Kiev government of committing acts of cruelty. It is said to consist largely of untrained locals and Russian volunteers.