Scientific communities have established social mechanisms for proposing explanations, questioning evidence, and validating claims. Opportunities like these are often not a given in science classrooms (Vellom, Anderson, & Palincsar, 1993) even though the National Science Education Standards (NSES, 1996) state that a scientifically literate person should be able to “engage intelligently in public discourse and debate about important issues in science and technology” (National Research Council [NRC], 1996). Research further documents that students’ science conceptions undergo little modification with the traditional teaching experienced in many high school science classrooms (Duit, 2003, Dykstra, 2005). This case study is an examination of the discourse that occurred as four high school physics students collaborated on solutions to three physics lab problems during which the students made predictions and experimentally generated data to support their predictions. The discourse patterns were initially examined for instances of concept negotiations. Selected instances were further examined using Toulmin’s (2003) pattern for characterizing argumentation in order to understand the students’ scientific reasoning strategies and to document the role of collaboration in facilitating conceptual modifications and changes. Audio recordings of the students’ conversations during the labs, written problems turned in to the teacher, interviews of the students, and observations and field notes taken during student collaboration were used to document and describe the students’ challenges and successes encountered during their collaborative work. The findings of the study indicate that collaboration engaged the students and generated two types of productive science discourse: concept negotiations and procedure negotiations. Further analysis of the conceptual and procedure negotiations revealed that the students viewed science as sensible and plausible but not as a tool they could employ to answer their questions. The students’ conceptual growth was inhibited by their allegiance to the authority of the science laws as learned in their school classroom. Thus, collaboration did not insure conceptual change. Describing student discourse in situ contributes to science education research about teaching practices that facilitate conceptual understandings in the science classroom.