Constructed between 1968 and 1976, the Baleares class began entering service in 1973 and formed the 31 Escort Squadron, based at Ferrol.

However, the negotiations failed as questions were raised in the United Kingdom over dealing with the unpopular Spanish government.

[1] Instead, the Spanish government turned to the United States and on 17 November 1964 an agreement acquire five Knox-class frigates was signed.

[2][3] As part of the agreement, the hulls and machinery were to be constructed in Spain at Ferrol while the weapons and sensors would come from the United States.

[2][a] The Baleares class were equipped with one Westinghouse geared turbine turning one shaft creating 26,000 kilowatts (35,000 shp).

[2][5] The frigates initially fitted AN/SPS-52A air search and AN/SPG-51C gun fire control radars along with AN/SQS-23 hull-mounted and AN/SQS-35A variable depth towed sonars.

[3] During the first stage modernisation, the ships were given AN/SPS-10F surface search radar and the AN/SPG-53 Mark 68 gun fire control system.