This document discusses the relation between adolescents' expertise (their informational background, which can be accurate or inaccurate, about a particular task or subject) and their formal operational reasoning, and reports the results of three studies of the effects of children's expertise on reasoning. In Study I, adolescents from a rural river delta area were compared with urban students in an experiential science program on three measures of the ability to control variables involving fishing, bending rods, and a third for which neither group had previous relevant experience. The river delta students performed better on the fishing task than on bending rods, the converse was found for the science students, and the two groups performed equally on the neutral task. In Study II, which involved high school seniors, a relationship was found between the ability to name variables in a particular problem and the ability to control them, and a relationship between field dependency and the ability to name variables. Study III examined the effects of prior expertise (accurate and inaccurate) and experience (increasing expertise) on the reasoning of seventh graders. Experience was found to influence reasoning, but the effects of experience were sometimes blocked by prior inaccurate expertise. These findings are discussed in connection with other relevant variables, e.g., field dependency and the question of whether or not to employ a given strategy (e.g., controlling variables). (BH)