This study assesses the structure of personality in two species of New World monkey. It addresses personality with comparison to other species of primate, and considers reasons for variation between species. It also considers the role of personality in social interactions. Data were collected from Questionnaire ratings of personality for a captive mixed-population of squirrel monkeys, Saimiri sciureus. These results were compared with data available for capuchins, Cebus apella, from the same study site. Data were also collected on the behavioural and social interactions of individuals. Principal Component Analysis for Cebus produced four factors, labelled Curious, Anxiety, Dominant and Attentiveness. For Saimiri, Principal Component Analysis and Regularized Exploratory Factor Analysis revealed three factors, Dominant, Neurotic and Sociable. Both species shared items on two factors, Dominant and Anxiety/Neurotic. Cebus were found to be more similar to Old World primates on Curious, which overlapped with item loadings on factors such as Openness and Extraversion. Attentiveness in capuchins appeared to be a unique trait. A preliminary analysis for behavioural data revealed species differences in behaviour, including allogrooming, lip smacking and levels of threats, dominant and submissive behaviour. Analysis of inter-rater agreement suggested improvements for behavioural data analysis in order to accurately assess correlations between behaviour, social interactions and personality. Current findings are discussed in light of intraspecific selection pressures and social structure.