This paper describes an empiric study of aggregation and deliberation used during citizens' workshops for the preference elicitation of 20 different ecosystem services (ESs) delivered by the Palavas coastal lagoons located on the shore of the Mediterranean Sea close to Montpellier (S. France). The impact of deliberation for the preference elicitation of 20 different ecosystem services (ESs) was studied by gathering and aggregating individual preferences before deliberation that were compared to the collective aggregation after deliberation. The same aggregation rules were used before and after deliberation and we compared two different aggregation methods, i.e. Rapid Ecosystem Services Participatory Appraisal (RESPA) and Majority Judgement (MJ). RESPA had been specifically tested for ESs, while MJ evaluates the merit of each item, an ES in our case, in a predefined ordinal scale of judgment. The impact of deliberation was strongest for the RESPA method. This new information acquired from application of social choice theory is particularly useful for ecological economics studying ES, and more practically for the development of deliberative approaches for public policies.