Despite the fundamental necessity of gaining gatekeeper approval for prisons fieldwork, researchers rarely publicly acknowledge and analyse their failures to secure access. Drawing upon the Foucauldian-inspired literatures on the production and policing of new criminological knowledge, this article presents as a case study the Sex in Prison research project, instigated by the Howard League for Penal Reform, and for which permission to interview serving prisoners was refused. This denial of access, it is argued, resulted from a politically motivated attempt to prevent the acquisition of knowledge perceived as likely to be unfavourable to and critical of prison authorities, emanating from ‘deviant’ informants, using ‘deviant’ methods and obtained on behalf of ‘deviant’ penal reformers. The troubling implications for prisons researchers are explored. The article concludes by highlighting the potential of formerly imprisoned people, as participants in research, to circumvent the limits to knowledge imposed by prison authorities.