The article analyses the mechanisms of denunciation used in the early years of operation of the Royal Inquisition court of Valencia in the late 15th century. Our study of the testimony given by a group of old Christians who denounced various Jewish converts in the city of Xàtiva has enabled us to reconstruct how denunciation took place, and the development of the climate of social betrayal which was encouraged by the inquisitorial authorities. We have identified the informants —old Christians— and those denounced —Jewish converts— and their respective socio-professional occupations. The informants were extremely intolerant of their Jewish converso neighbours, whose attitudes they monitored. They used the new horizontal mechanisms for social control, implemented, disseminated and encouraged by the inquisitorial authorities, which encouraged old Christians to observe, monitor and report dissident behaviour and practices among Jewish converts in Valencia.