This train is indeed notoriously difficult to reserve. You should book these passholder reservations as soon as you arrive in Spain. There is no passholder quota on this train, so as long as there are seats available you should be able to get a reservation.

Unfortunately, July 4 might be a peak date, so I can't guarantee anything. For full peace of mind, you might want to reserve seats on the TGV to Nîmes (€25, can be fully refunded up to shortly before the departure). Alternatively there are reservation-free rputes possible, but those take about 2-3 hours more.

Thank you for the tips, so I gather a passholder reservation online is not possible prior to arriving in Spain?

Yes not possible at all (at least for now). I second the TGV suggestion:

Book seats until Nîmes for 25€ pp and if by chance you can get reservations for the direct service, cancel them -> easily doable through Rail Europe website.

Itinerary would look as follows:

- TGV Barcelona-Sants - Nîmes 13:25 - 17:15

- TER Nîmes - Marseille St-Charles 18:20 - 19:40

Thanks - I can’t get a passholder fare on Rail Europe for the TGV Nimes route either, it says that a rail pass does not apply. Can only get the full fare of 64 Euro. This has been happening for a few routes on Rail Europe of late, where no passholder fare available e.g. Lausanne to Paris. Not sure why??

They appear as available when I look:

okay, not sure why it won’t show me global passholder fares as I’m entering it in the search, will keep trying, chrs

Are you using the desktop version and selecting the pass in the start screen? The option does not appear on the mobile website.

Yes, logged in on desktop and adding the rail pass before searching. It recognises it for some routes but not others, such as this one.