A study is made of the segregation effect of adsorbed species related to the inner structure of two bidimensional adsorbent fractals, the incipient percolation cluster (IPC) and the backbone of the IPC, and their connection with the rate-determining step of the kinetic mechanism of the monomer-dimer (MD) surface reaction on those fractals. Production, R-AB, is proportional to the concentration of A in the gas phase gamma(A) and to the fraction of vacant superficial sites x(E) (R-AB = gamma(A)x(E)), and it is shown that adsorption of the monomer is the rate-controlling step of the reaction mechanism. (C) 2003 Elsevier Inc. All rights reserved.