A new measure, Partner Behaviours as Social Context (PBSC), was developed and tested in two studies (N = 215, N = 316) and tested as a correlate of psychological wellbeing in a third (N = 157). Self-Determination Theory (Deci & Ryan, 2000) and related research has suggested that there are six important dimensions of partner behaviours, which should be associated with individual psychological need fulfilment, wellbeing and development. Three of these dimensions are positive, including warmth, or provision of love/affection; autonomy support, or supporting a partner's decisions; and structure, or being consistent and reliable. Three of these dimensions are negative, namely rejection, or ignoring/being hostile to a partner; coercion, or being controlling/demanding; and chaos, or unpredictability. Factor analysis supported the six-dimensional construct. The six PBSC dimensions, positive and negative subscales and total score had good distributional properties, high internal consistency, related as expected to other relationship quality measures, and diverged from most personality constructs. All PBSC scales were related to wellbeing, with some behaviours more related than others. For example, coercion was strongly associated with compromised wellbeing, while warmth showed weak, positive associations with positive functioning. The PBSC is expected to have utility in both research and clinical settings.