The aim of this study is to apply some ideas from Charles S. Peirce's theory of signs to the analysis of counselor behaviors during motivational interviewing sessions in the probation service by focusing on the counselors' interpretations of their substance-abusing clients' change talk. The results of the study display that making the argument interpretation is the most certain way to make the client talk for the change and to help the client to reach the goal with regard to his/her use of alcohol and drugs. The argument interpretation shows something that the client has not noticed before (deductive argument), captures an anomaly that appears in the client's talk (abductive argument) or makes a conclusion of the client's situation (inductive argument) and presents an assertive inference based on them. The results show that by applying Peirce's theory of signs, we can find the new features of client–counselor interaction that potentially relate to the treatment outcome.