Equilibrium adsorption was used to study the influence of some parameters on the precursor formation of catalysts consisting of zirconia-supported chromia (ZC) or molybdena (ZMo). Uptake capacities of zirconia treated at various temperatures before adsorption (from 673 to 1023 K) showed little dependence on temperature. The Me uptake (Me = Cr or Mo) as a function of the equilibrium concentration of Me species in solution obeyed the Langmuir law for limited ranges. The Langmuir plots therefore yielded saturation coverage values, n(m), that depended on the concentration range. The meaning of "monolayer coverage" is discussed. The n(m) value measured at a range of solution pH from 1 to 8 depended little on pH. The values ranged from 1.6 to 2 atoms nm(-2) for Cr, and were around 3.6 atoms nm(-2) for Mo. The adsorption data suggested three adsorption regimes : (i) a strong, apparently irreversible adsorption, (ii) a partly reversible, direct adsorbate-surface interaction, corresponding to the initial stage in the Langmuir plots, which eventually merges in (iii), a weaker adsorption regime, showing an adsorbate-adsorbate interaction. We compare our data with reported values for Cr and Mo equilibrium adsorption on alumina.