A phenomenological study of the phase transitions occurring in the adsorption systems Pb/Ge(111) and Sn/Ge(111) is presented. The starting point of such a study is the Landau theory. The critical behaviour expected theoretically for the two interfaces, and the corresponding influence of defects, are discussed in detail. Symmetry arguments show that, contrary to general belief, the critical behaviours of Pb/Ge(111) and Sn/Ge(111) are essentially different. The Landau-like approach employed to study the influence of defects provides a consistent and general manner to interpret the existing experimental data. Special attention is paid to the influence of hopping defects in Sn/Ge(111).