Anodic dissolution of aluminum in hot chloride solutions produces a high density of fine etch tunnels that extend along [100] directions. Tunnels evolve from cubic etch pits when all but one of the pit wall surfaces become passivated; dissolution then occurs at the one active face at a rate that may initially be as high as 20 A/cm2. Tunnels have square cross sections with sides ~1 μm and aspect ratios as high as 100:1. Tunnel growth may be considered a unique form of pitting corrosion in which dissolution and passivation occur simultaneously with a sustained balance between the two processes.