One effect of the policy of deinstitutionalisation has been to increase police contact with people, who are experiencing the effects of acute mental illness. Policy documents such as Home Office circular 66/90 recognise that adults with mental health problems are especially vulnerable within the criminal justice system. The overall aim of policy is that vulnerable adults should be diverted to mental health services at the earliest opportunity unless the offence is so serious that this would not be in the public interest. However, there is little concrete evidence of the success of this policy. The result is that police officers have an increasing role to play in working with individuals experiencing acute mental health problems. In this process, custody officers have a key role to play as decision‐makers as to whether the protections that PACE (1984) offers to vulnerable adults should apply. This article is based on a small‐scale indicative research study, which examined how officers make these decisions and the training that they receive relating to mental health issues.