Current research and learning theory suggest that a hierarchy of proportional reasoning exists that can be tested. Using G. Vergnaud's four complexity variables (structure, content, numerical characteristics, and presentation) and T. E. Kieren's model of rational number knowledge building, an epistemic model of proportional reasoning was constructed and tested with 1 male and 17 female preservice elementary education majors. A 12-item test of proportional reasoning was administered, followed by individual 30-minute interviews to probe general and specific reasoning and problem-solving approaches of students on a variety of proportional reasoning tasks. Data were collected in the fall of 1992. Item difficulty levels were calculated using a scoring rubric developed to take into account a variety of complexity and structural characteristics of the problems. Simple linear regression analysis revealed that the calculated scores did predict student performance with item difficulty accounting for approximately 30 percent of the variance of student performance on test items. Results support the model of a proportional reasoning construct consisting of levels of proportional reasoning and including contextual and problem features as relevant to success in performing proportional reasoning tasks. Understanding the levels of proportional reasoning may guide decisions concerning instructional sequencing and content. Seven figures are provided. (Contains 20 references.) (Author/SLD)