Founded on a treaty of 1113 between the Republic of Pisa and Ramon Berenguer III, Count of Barcelona, the expedition had the support of Pope Paschal II and the participation of many lords of Catalonia and Occitania, as well as contingents from northern and central Italy, Sardinia, and Corsica.

[2] In September 1113 a Pisan fleet making an expedition to Majorca was put off course by a storm and ended up near Blanes on the coast of Catalonia, which they initially mistook for the Balearics.

[3] The Pisans met with the Count of Barcelona in the port of Sant Feliu de Guíxols, where on 7 September they signed a treaty causa corroborandae societatis et amicitiae ('for strengthening [their] alliance and friendship').

Specifically the Pisans were exempted from the usagium and the jus naufragii in all the territories, present and future, of the Count of Barcelona, though Arles and Saint-Gilles, in the recently acquired March of Provence, were singled out for special mention (three times).

[2] Paschal had also granted the Pisans the Romana signa, sedis apostolicae vexillum ("Roman standard, the flag of the apostolic see"),[10] and his appeals for the expedition had borne fruit.

[14] As the siege dragged on the counts of Barcelona and Empúries entered into peace negotiations with the Muslim ruler of Majorca, but the cardinal and Pietro Moriconi, the Archbishop of Pisa, interfered to put an end to the discussions.

Probably the Catalan rulers, whose lands lay nearest the Balearics, expected an annual payment of parias (tribute) from the Muslims and the cessation of pirate raids in return for lifting the siege.