Barton waits for Mbia agreement with QPR
Barton passed a medical in Marseille after arriving on Wednesday and took part of a light training session on Friday morning.
But the 29-year-old midfielder will only join the French side if Cameroon defender Mbia, who has flown to London, completes his move to the Premier League.
"The two clubs have reached an agreement. Mbia still has to find one with QPR. Then Joey will be here," Marseille's sports director Jose Anigo told reporters.
"Joey's arrival is linked to Mbia's move to QPR. It will be a transfer for Mbia and a loan for Joey.
"There are quite good chances that the deal will be completed. But it's not done yet."
Barton has been frozen out of QPR's plans after his sending-off against Manchester City in the final game of last season resulted in a 12-match ban.
Mbia joined Marseille from Stade Rennes in 2009 and was key in their successful run to the French league title in 2010.
The 26-year-old, who has 42 caps for Cameroon, can play either as a midfielder or centre-back.