[en] The propagation velocity of the He II-He I interface at 1 atm was measured in a large experimental cryostat. During cooldown, He II at the lambda temperature propagates up and down in a gravitational field with an interface velocity V/sub b/ which depends on the He I temperature T/sub infinity/. For T/sub infinity/ = 4 K, V/sub b/ = 0.1 mm/sec and for T/sub infinity/ = 2.2 K, V/sub b/ = 2 mm/sec. During warm up, He I propagated with an interface velocity V/sub b/ > 30 mm/sec that was measured as low bound only. A mathematical model gives a temperature dependent interface thickness in the range 2 x 10^-3 approx. = delta < 4 x 10^-1 mm