A Frenchman and a Colombian-Italian will represent Europe on an 18-month simulated mission to Mars.
Romain Charles and Diego Urbina have been chosen to go into a set of steel containers on 3 June with two Russians and a Chinese national.
The group's exile will test the physical and mental requirements of ultra-long duration spaceflight.
Their Mars500 "spaceship", which is located in Russia's Institute of Biomedical Problems, has no windows.
All the food and water needed for their "journey" will have to be loaded before "departure".
The experiment's designers intend to make the exercise as realistic as possible by introducing a time delay in communications after two months.
Because it can take about 20 minutes for a message to travel from Mars to Earth, it will take this amount of time in the simulation, also.
Thirty-one-year-old Romain Charles works for a company producing composite panels. Twenty-six-year-old Diego Urbina is an electronics engineer.
The remaining members of the all-male crew will be chosen in the coming days.
The Mars500 facility comprises four sealed and interconnected modules. A further container represents the Martian surface.
After about 250 days, half the crew will enter a "landing craft" and "descend" to the planet where they will walk about wearing modified Russian Orlan spacesuits.
Scientific investigations during the experiment will assess the effect that isolation has on various psychological and physiological aspects such as stress, hormone levels, sleep quality, mood and the benefits of dietary supplements.
The organisers say that if a crew-member decides halfway through the simulation that he really cannot cope with the separation anymore and wants to leave, he will be allowed to do so; although every effort will be made to try to persuade the individual to stay.
Romain Charles and Diego Urbina were chosen in a selection process run by the European Space Agency (Esa) which is supporting the Moscow-based experiment.