Based on previous research on mental-accounting (Thaler, 1999; Kahneman & Tversky, 1984) and recent studies on process frames in negotiations (Trötschel, Loschelder, Höhne, & Majer, 2015) a decision-making process model of mental accounting in negotiations is introduced. The research project seeks to empirically test this model. In the model five conceptually delineated steps of mental accounting are described: 1) mental parsing, 2) mental budgeting, 3) mental balancing, 4) outcome editing, and 5) mental closing. The empirical test of the model and its different steps may lead to new insights within the negotiation and the decision-making literature: From the perspective of negotiation research a new theoretical approach is introduced that helps to investigate psychological processes that have not been considered so far (e.g., mental parsing, mental balancing). Moreover, the model offers a theoretical framework for different psychological processes that have been considered to be unrelated (e.g., process-framing, agenda-setting). From the perspective of decision-making research a well-established concept of consumer research will be tested in a context that allows for the social interaction between the decision-makers. Thus, combining two different lines of research may lead to new theoretical and practical perspectives on decision-making research in general and negotiation research in particular.