Voluntariness of consent to research has not been sufficiently explored through empirical research. The aims of this study were to develop a more comprehensive approach to assessing voluntariness and to generate preliminary data on the extent and correlates of limitations on voluntariness. We developed a questionnaire to evaluate subjects’ reported motivations and constraints on voluntariness. 88 subjects in five different areas of clinical research—substance abuse, cancer, HIV, interventional cardiology, and depression—were assessed. Subjects reported a variety of motivations for participation. Offers (...) of financial incentives were common but not influential, pressures from others were rare, and no threats were reported. However, certain financial incentives and—paradoxically—altruistic motivations led some subjects to feel more constrained. Consistent with previous studies, no one pattern of motivation was common to all research subjects. There was little evidence of constrained voluntariness, but some suggestion of areas of concern. Voluntariness appears to be susceptible to systematic empirical investigation. (shrink)