Variations of the concentration of water components (vapor + mist) at the outlet of an open-column cold trap with time are determined under different gaseous streams of N-2, He, and H-2. Ice mist generated in the trap is deposited from a laminar convection flow in a vertical circular tube onto a wall surface cooled by liquid nitrogen. The mist deposition rate from the He or H-2 stream is greater than that from the N-2 Stream at the same volumetric flow rate and inlet water vapor pressure. This is because a thermophoretic force acts on mist particles more strongly in the He and H-2 streams than in the N-2 stream. The experimental mist deposition rate is compared with calculations using previous theoretical equations on thermophoresis. The open-column cold trap cooled by liquid nitrogen shows good performance to remove ice mist almost completely from gaseous streams with a comparatively small velocity.