This paper deals with a comparative LEED-AES study of segregation of tin and lead on (111) surfaces from Ge(Sn) and Ag(Pb) solid solutions, respectively. The main feature of this work is the experimental observation of stepped kinetics in both cases even though the structures of the (111) surface in cubic diamond (Ge) and in fcc (Ag) lattices differ largely. This result is related to the occurrence and the stability of successive overlayers during the monolayer formation. Comparison with previous studies of the first stages of formation of ultra-thin layers of Sn and Pb onto Ge(111) and Ag(111) surfaces was carried out. The observed 2D phase transitions are explained by considering the energy of the different superficial sites which the segregated atoms can occupy.