Among them were enlargement of the false tower at the top, more modern engines, installation of a radiogoniometer that could be controlled from inside the ship.

Iride and Onice were "lent" to the Nationalist side during the Spanish Civil War, under the names of Gonzales Lopez and Aguilar Tablada respectively, in the framework of Italy's aid to Franco's regime.

At the outbreak of hostilities, four boats were located at La Spezia, three in Cagliari, two in Augusta and one, Perla, in Massawa.

In July 1940 Iride was modified to carry 4 "Maiale" manned torpedoes in watertight containers on the deck of the submarines.

In December 1942, Ambra managed to penetrate Algiers harbor, and sank or seriously damaged several merchant ships with a total GRT over 20,000.

They showed good maneuverability, their hull was well designed and strongly built to withstand depth pressure and explosions that exceeded their test values.