The combined dry-steam reforming (CDSR) of a model biogas (66 % CH4 + 34 % CO2) has been investigated under various feed compositions and reaction temperature conditions over a Ni-Rh/MgAl2O4 catalyst, with the objective to convert the excess of methane (compared to CO2) by steam reforming (SR). Methane, however, appeared to react preferentially with H2O rather than with CO2, leading to an inhibition of the dry reforming (DR) reaction in the presence of steam. CO2 and H2O adsorption and co-adsorption studies revealed that H2O was always adsorbed in much higher amounts than CO2 on the catalyst surface. A Temkin adsorption model applied to temperature-programmed adsorption experiments (TPAE) showed that two different strongly adsorbed CO2 and H2O species were present on the catalyst surface at moderate temperature, but only one of each remained adsorbed at temperatures relevant to the reaction conditions (600-800 degrees C). H2O was also shown to displace CO2 from the catalyst surface, confirming that they compete, at least partially, for the same adsorption sites. At high temperature, the surface coverage by the most strongly adsorbed H2O species was predominant. These results should contribute to the development of efficient catalytic processes for biogas valorization to produce syngas and hydrogen from renewable sources.