Here is some information on the elusive and arcane "Companion Voucher".

During 2023, Eurostar and Thalys merged. This led to a complete overhaul of the points and benefits system. One major change was the introduction of the "Etoile" tier, above Carte Blanche. this lists "Companion Vouchers" as a benefit exclusive to Etoile members. I asked about it in the lounges, on the train and even at the ticketing counter - nobody knew what it was. So I decided to call up Eurostar customer service, and they were able to help. It took about 10-15 minutes for the agent to go and ask her colleagues (or management) for the details - she explained that some people confused it with a "companion ticket" for disabled passengers, or a "voucher" issued for other reasons.

What is it?

• The Eurostar Etoile "Companion Voucher" allows a member with a points ticket in Standard or Standard Premier (not Business Premier) to add another traveller to a single journey (not a return) for free.
• Two such vouchers are issued per Etoile account, per membership year.

How does one redeem it?

• Step 1. Book a journey on points, in Standard or Standard Premier.
• Step 2. Call Eurostar customer services and tell them that you would like to utilise your Etoile Companion Voucher, and have them apply it to the (single or return) journey.

I've not yet tried this, but thought I'd pop the information up here, since I cannot find this anywhere else online and I felt that, without knowing what it is, some people might be at risk of letting their voucher lapse in Sep/Oct when the membership year resets.