9 pages., Via online journal., Lack of trust is thought to be one of the most significant barriers to the consumption of organic foods, which is an important dimension of sustainable behaviour. Building trust in organic foods is the central objective of this paper. Based on information processing models focusing on what message to transmit and how, and on the premise that to improve trust, two different dimensions (functionality and authenticity) must be managed simultaneously, this paper analyzes the comparative effectiveness of different combinations of message arguments, forms of appeal and sources on consumer trust. To this end, an experiment was designed with a total of 800 participants, in which 36 different treatments were tested. The results show strong interactions between the three variables considered and suggest that the most effective combinations for building trust are: the health argument put across by an expert, the authenticity argument transmitted by a producers’ union, the elitist argument made by an expert and lastly, the social argument transmitted by a public authority, using an emotional form of appeal in all four cases. These results serve to complete the previous literature on the subject, in which communication activities are recommended but the questions of what to say, how to say it and who should say it are not specifically addressed.