“Chris Peterson’s unfinished masterwork: The real mental illnesses” offers a theoretical foundation describing psychological pathology from the perspective of character strengths (Seligman, 2013, 2015). Within this framework, character strengths represent personality-related indicators of mental health, whereas the absence, excess, or opposite of character strengths indicate mental illness. Inspired by Seligman’s memorial address on Chris Peterson at the WCPP in 2013, we started to work on making Peterson’s masterwork applicable for empirical research. We firstly focused on the identification and definition of the opposites: We (1) adapted the criteria that define character strengths (Peterson & Seligman, 2004) to be applicable to define opposites, (2) identified terms for literature research by utilizing antonym dictionaries, and (3) searched literature regarding already existing psychological constructs related to those terms. Consequently, we identified and defined 24 candidates of possible opposites. Furthermore, we developed a measure assessing these opposites. Initial results showed, the opposites were (1) moderately negatively correlated with the character strengths, (2) positively related to indicators of mental illness (e.g., anxiety, depression), and (3) negatively associated with indicators of mental health (e.g., psychological well-being). We will discuss the current status of our project (e.g., definition and measurement of excesses), limitations, and next steps.