Introduction and Aims. Despite large increases in pharmaceutical opioid dependence and related mortality, few studies have focused on the characteristics and treatment experiences of those with pharmaceutical opioid dependence. We describe the formation of a prospective cohort of people receiving treatment for pharmaceutical opioid dependence and describe their baseline characteristics. Design and Methods. People who had entered treatment for pharmaceutical opioid dependence (n = 108) were recruited through drug treatment services in New South Wales, Australia. We describe baseline characteristics of those that commenced pharmaceutical opioids for pain or other reasons and conducted a thematic analysis of responses regarding their treatment experience. Results. Mean age was 41 years (SD 11), half were male (48%). Just over half reported lifetime heroin use (57%). Oxycodone (49%) and codeine (29%) were the most common opioids reported. Most (85%) reported past-year problematic pain, 38% reported chronic pain. Half (52%) reported moderate to severe depression symptoms. Most (66%) commenced opioids for pain, and this group were older, less likely to report a previous overdose and less likely to report use of illicit drugs compared to those commencing for other reasons. Five themes related to treatment expectations: (i) stigma; (ii) the restrictive nature of treatment; (iii) knowledge; (iv) pain; and (v) positive experience with buprenorphine. Discussion and Conclusions. This study describes the complexities in an important emerging treatment population of pharmaceutical opioid-dependent people. Findings highlights that addressing knowledge and perceptions around treatment may be critical to address the rising mortality associated with pharmaceutical opioid dependence. [Nielsen S, Lintzeris N, Murnion B, Degenhardt L, Bruno R, Haber P, Johnson J, Hardy M, Ling S, Saddler C, Dunlop A, Demirkol A, Silsbury C, Phung N, Houseman J, Larance B. Understanding an emerging treatment population: Protocol for and baseline characteristics of a prospective cohort of people receiving treatment for pharmaceutical opioid dependence. Drug Alcohol Rev 2018;37:887-896]