The paper examines conversations in the 18th-century London and Paris financial marketplaces. The aim is to highlight the place of conversations as the key form of interaction in the marketplace, and to evaluate financial conversations against the broader cultural background of literary and scientific dialogues of the time. The relevance of this enterprise is that it leads to a better understanding of how the verbal interactions of the marketplace shape transaction outcomes and contribute to forms of rationality specific for financial markets. Grounded in the analysis of empirical material, the paper distinguishes between conversations-qua-transactions and conversations-about-the-world. It shows how they produce and require specific forms of knowledge from the participants; at the same time, they shape the transactions' outcomes. On this basis, the paper argues that the phenomenon of sudden mood swings in the marketplace cannot be entirely explained in irrational, psychological terms, but must be seen as the outcome of a particular conversational system.