Mr Gonzalez convulsed the party's congress in Madrid on Friday by unexpectedly saying that he would not stand again.
However, Mr Gonzalez's example, intended to inject new life into the leadership, was not followed by the powerful regional barons who insisted on clinging to their seats. It was with their blessing that Mr Almunia emerged as the successor.
Mr Almunia, 49, a Basque from Bilbao who joined the party in Madrid in 1974 and became adviser to the socialist General Worker's Union (UGT), was elected to parliament in 1979, when he joined the party executive.
He became Minister of Labour and Social Security in Mr Gonzalez's first government in 1982. At 34, he was the youngest minister in that youthful government.
He has long combined positions in the party with those in public political life, and in the last year has been Mr Gonzalez's' effective and energetic right hand in parliament.
Mr Almunia has carried the weight of leading the socialist opposition's fight against the conservative government of Jose Maria Aznar, since Mr Gonzalez has kept his own parliamentary appearances to a minimum. Trained in law and economics at Bilbao's Jesuit Deustuo University, and at the Ecole des Hautes Etudes in Paris, Mr Almunia is known for his intellectual astuteness.
He is also a safe pair of hands, who, although recognising the need to make changes to restore the party's vigour, is unlikely to turn everything upside down.