The purpose of this study was to develop and validate a comprehensive Content Analysis Manual (CAM) as an instrument for the systematic, a priori analysis of closing defense arguments in the penalty phase of a capital trial. Only two previously published studies included a content analysis of penalty-phase summation transcripts, and those were a posteriori studies in which researchers conducted an exploratory analysis to identify commonly used argument themes, creating variables for those themes after the fact (Costanzo & Peterson, 1994: Logan, 1983). To date, no a priori content analysis of penalty-phase summations—in which all variables are defined and operationalized through coding rules before text is analyzed—has been published. The CAM in this research project was developed based on closing-argument literature, court transcripts of penalty-phase summations, juror interview studies, and social psychology literature on persuasion. The manual includes an exhaustive list of penalty-trial argument points and persuasion techniques commonly used by defense attorneys. Instrument reliability was evaluated through two studies in which three coders independently analyzed closing argument transcripts obtained from the files of the California Appellate Project. Data for argument points (AP units) were coded and analyzed separately from data for persuasion techniques (PT units). Based on data assessment and coder comments from Study 1, revisions were made to the CAM prior to Study 2. For Study 2, the average between-coder agreement for unit location was 87% for AP units (n = 248) and 82% for PT units (n = 181). Data for agreement of code choice resulted in an average Cohen's kappa coefficient of .750 for AP units (n = 248) and .737 for PT units ( n = 181). These results indicate that the CAM is a valid instrument for a priori analysis of penalty-phase defense summations, which will facilitate further research on the construction of more persuasive closing arguments.