The internal porous structure of a chromia-promoted skeletal cobalt catalyst was directly and clearly imaged for the first time. The structure was made finer by the addition of sodium chromate to the leaching solution during preparation, and this resulted in a higher BET (Brunauer, Emmett, and Teller) surface area. Leaching proceeded with a sharp reaction front and followed an Avrami-Erofe'ev kinetic equation. A full quantitative model including parameters for particle size, chromate concentration, and temperature was derived. The activation energy of 75 kJ mol(-1) was not significantly different from that for unpromoted cobalt. The mechanism of producing the finer structure in light of the kinetic information is discussed in, terms of the exposed aluminum surface area.