This paper describes a study of mass-transfer process and particle motion in a plating barrel. The mass-transfer coefficient was measured with a diffusion-controlled metal-dissolution reaction, and the motion of metal particles was examined with a video recording system. Three kinds of particle motion were observed in a horizontal barrel : (i) slumping motion at a low barrel rotational speed of 3 rpm; (ii) falling motion at the barrel rotational speeds of 6 to 13 rpm; and (iii) cascading motion at 15 to 17 rpm. When the barrel was tilted at an angle of 30 to 60 degrees from the horizontal position, the particles in the lower portion of the barrel rotated like a rigid body. There was no relative particle movement in this regime. The falling motion of particles occurred only near the top surface of the particle load. At a barrel tilt angle of 90 degrees (i.e., a vertical barrel), all the particles rotated like a rigid body, and Ilo falling layer was observed. The mass-transfer rate to the particles increased with increasing barrel rotational speed, and decreased with increasing tilt angle from the horizontal position. When the barrel tilt angle was less than 60 degrees, the mass-transfer rate decreased with increasing barrel loading. The effect of barrel loading on mass transfer decreased with increasing tilt angle; at a barrel tilt angle of 90 degrees the barrel loading had a negligible effect on the mass-transfer rate in the barrel. A set of empirical equations was obtained to correlate the Sherwood number to the Reynolds number, Schmidt number, Grashof number, barrel tilt angle, barrel loading, and barrel immersion.