The present article investigates the effectiveness of methods traditionally used to distinguish between the emotions of regret and disappointment and presents a new method --- the Regret and Disappointment Scale (RDS) --- for assessing the two emotions in decision making research. The validity of the RDS was tested in three studies. Study 1 used two scenarios, one prototypical of regret and the other of disappointment, to test and compare traditional methods (``How much regret do you feel'' and ``How much disappointment do you feel'') with the RDS. Results showed that only the RDS clearly differentiated between the constructs of regret and disappointment. Study 2 confirmed the validity of the RDS in a real-life scenario, in which both feelings of regret and disappointment could be experienced. Study 2 also demonstrated that the RDS can discriminate between regret and disappointment with results similar to those obtained by using a context-specific scale. Study 3 showed the advantages of the RDS over the traditional methods in gambling situations commonly used in decision making research, and provided evidence for the convergent validity of the RDS.