This study investigated the grouping effect on student achievement in a chemistry laboratory when homogeneous and heterogeneous formal reasoning ability, high and low levels of formal reasoning ability, group sizes of two and four, and homogeneous and heterogeneous gender were used for grouping factors. The sample consisted of all eight intact chemistry classes of a medium-sized high school located in a university town in the southeast. Five laboratory experiments were conducted in each class. These experiments were organized to be open-ended so that procedure and work assignments could be determined by the group. The experiments covered the following topics: (1) indirect evidence; (2) density of liquids; (3) physical properties of ionic and covalent compounds; (4) freezing point determination; and (5) classification of common acids and bases. Included in this paper are the introduction and rationale, a literature review, the research protocol, results, conclusions, implications, and recommendations. (KR)