A study examined the way argumentation in collective bargaining shapes outcomes. Specifically, it explored the types of claims and reasoning processes that characterize bargaining interaction on different subissues of a proposal and tracked the development of arguments through sequential sessions and caucus meetings by examining similarities and differences in the reasoning process of subissues that were dropped, modified, or retained in the final agreement. Data were collected through extensive observations of bargaining and caucus sessions that occurred during negotiations between a school board and teachers' union, and from interviews, questionnaires, and document analysis. Although the reasoning process differed across subissues, some general conclusions emerging from the study were as follows: (1) the most prevalent types of claim used in bargaining were evaluative and definitive; (2) both sides employed reasoning from analogy, cause, and hypothetical example more frequently than they did other types of reasoning; (3) the reasoning process appeared more complete with the use of evaluative claims than with definitive, factual, and policy ones; and (4) proposals and subissues changed, not through adding more information, but through shifting types and adding qualifiers. (FL)