This study forms part of the South African Personality Inventory project that aims to develop: (a) an indigenous theoretical model of personality; and (b) a unique personality measure that is in line with South African legislation and that can be used fairly to assess personality across different South African language and cultural groups. In line with this mandate, the objectives in this study were twofold: first, to validate the Relationship Harmony and Soft-Heartedness Scales of the South African Personality Inventory and to determine whether these constructs are unique and distinct from the Big Five personality factors, and second, to determine whether Relationship Harmony and Soft-Heartedness are predictive of an external criterion, prosocial behaviour. A quantitative, cross-sectional research design with convenience sampling was used. Data were gathered from students (N = 431) at two tertiary institutions in South Africa, using preliminary Relationship Harmony and Soft-Heartedness Scales, the Basic Traits Inventory (Short form: Research Version), and the Prosocialness Scale. Results indicated that the Relationship Harmony and Soft-Heartedness Scales yielded five and six reliable factors, respectively, and presented good content, convergent, discriminant, and predictive validity, adding substantial predictive value for prosocial behaviour over and above the Big Five Model. Keywords:Five-Factor Model, personality assessment, prosocial behaviour, Relationship Harmony, Soft- Heartedness, South African Personality Inventory.