Two separate regulatory regimes govern research with adults who lack capacity to consent in England and Wales: the Mental Capacity Act (MCA) 2005 and the Medicines for Human Use (Clinical Trials) Regulations 2004 (“the Regulations”). A service evaluation was conducted to investigate how research ethics committees (RECs) are interpreting the requirements. With the use of a coding scheme and qualitative software, a sample of REC decision letters where applicants indicated that their project involved adults who lacked mental capacity was analysed. The analysis focuses on 45 letters about projects covered by the MCA and 12 letters about projects covered by the Regulations. The legal requirements for involving incapacitated adults in research were not consistently interpreted correctly. Letters often lacked explicitness and clarity. Neither consent nor assent from third parties is a legally valid concept for purposes of the MCA, yet they were suggested or endorsed in 10 post-MCA letters, and there was evidence of confusion about the consultee processes. The correct terms were also not consistently used in relation to clinical trials. Inappropriate use of terms such as “relative” had the potential to exclude people eligible to be consulted. Unless the correct terms and legal concepts are used in research projects, there is potential for confusion and for exclusion of people who are eligible to be consulted about involvement of adults who lack capacity. Improved clarity, explicitness and accuracy are needed when submitting and reviewing applications for ethical review of research in this area.