The four members of staff, who all work at Brieva prison in Ávila, are due to be questioned over the sexual abuse of seven female inmates.
The first complaint against a member of prison staff was made back in November 2013, when a female inmate complained about being forced into sex. The case was brought to court in Ávila, but the woman later retracted her statement.
In early 2014, another complaint was logged. This time a Brazilian prisoner said a staff member had suggested a threesome after he had sexually abused her girlfriend.
Spanish daily newspaper El Mundo described the case of one Brazilian inmate, 'C', who was told, after serving half her sentence in Spain, that she could return to Brazil to serve out the rest of her jail term.
After being abused, however, she was informed that she would have to stay in Spain to take part in the trial; her hopes of returning home to Brazil were dashed. In the worst case, she will retract her statement in order to return home to Brazil.
There appears to have been a bribe system at work in the Ávila prison, with guards offering perfume, alcohol and phone cards in exchange for sex.
One inmate admitted having sex with a member of prison staff in exchange for chocolate, while another was given €80 ($90) in exchange for sex.
Prisoners claim the four guards each had a different style of wooing the inmates. While one was friendly, bringing the women presents in exchange for sexual favours, two others were stricter, forcing inmates into sex.
The four members of staff will appear before a judge for questioning on Friday in Ávila.