The Socialists are hoping for a high turnout, which would be another valuable boost after it has won valuable coverage ahead of the 2012 presidential campaign.
There are 9,474 polling stations in schools, town halls and rented rooms throughout mainland France. Anxious not to be caught short, the party has printed 7.5 million ballot papers for each candidate so that voters can pop the one of their choice into the box.
Polliing closes at 17.00 GMT. Results will be posted as they come through on the official website. A final result is expected between 19.30 and 20.00 GMT.
In the first primary vote 2,665 million people chose between five candidates, putting Hollande ahead with 39.17 per cent and Aubry second with 30.42 per cent.
Hollande is now favourite since all four of the other challengers rallied behind him earlier this week.
But Aubry, who performed well in the last televised debate on Wednesday, has not given up hope. In an attempt to narrow the gap with Hollande over the last week she portrayed her rival as “soft left”, so she must have been particularly disappointed when Arnaud Montebourg, generally seen as the most left-wing of the
candidates-to-be-candidate, announced that he will be voting for Hollande.
As in last Sunday’s first round of the primary, anyone on the electoral rolls can vote if they sign a declaration of vaguely left-wing principles and, if they did not do so last time, pay one euro.
Supporters of both candidates were leafleting shopping centres and markets on Saturday.
In Clichy, north of Paris, potential voters were debating their options.
Hollande has "opened the door to practically everyone on the left”, one local resident told RFI.
Sadia, a woman of Moroccan origin voting for the first time in France, and Nestor, from Congo-Brazzaville, were attracted by his promise to restore 60,000 teaching posts cut under President Nicolas Sarkozy.
In the opposite camp, 40-something Lawrence, declared "I'm voting Aubry for the firm, straight, line she's kept to.”
Even if an opinion poll in Sunday’s Journal du dimanche newspaper showed 61 per cent feeling that there has been too much media coverage of the primaries, the four televised debates and attendant buzz have squeezed the other parties out of the pre-electoral debate over the past few weeks.
Following a setback in the Senate and poor poll showings, Jean-François Copé, the head of Sarkozy’s UMP, tried to rally the right-wing troops at the weekend, promising a “counter-offensive” after “this interminable sequence of the Socialist primaries”.