Aims: Validated schedules such as the Treatment Perceptions Questionnaire or Patient Satisfaction Questionnaire quantify service user satisfaction levels as a quality of service measure. This study assesses whether the validated survey instrument SERVQUAL can be used to measure service user satisfaction levels of the Integrated Drug Treatment System (IDTS) in English prisons. Methods: In this study, 96 face-to-face prisoner interviews were undertaken using SERVQUAL across eight prisons in England. Expectations, perceptions and a gap analysis were undertaken. Statistical reliability and validity of SERVQUAL were tested using principal components analysis and service quality factors were examined using linear regression methods. Findings: Prisoners were shown to have high expectations of IDTS services. Apart from one domain (tangibles), prisoners perceived services as not meeting their expectations. Factor analysis demonstrated that SERVQUAL had poor construct validity as the five domains did not map as expected. Three new service quality factors were identified - interpersonal, treatment content and professional qualities - along with an inverse relationship with previous treatment experiences prior to prison. Conclusions: Perception-only schedules may mask dissatisfaction levels by excluding the role that expectations play in the treatment process. More nuanced schedules are required to quantify satisfaction levels with reference to previous treatment experiences. Interpersonal qualities and treatment content qualities should form domains that measure service users' satisfaction with treatment.