Shorttitle This study investigated factors that may predict members'' commitment toward their unions. In this respect, this study focused on the relationship between personal factors, union factors, organizational factors and union commitment dimensions (affective attachment and instrumental attachment) among union members in Malaysia. This study also examined the influence of the union sector as a moderator on the relationship between the factors and union commitment. The members were chosen from AUEGCAS (public sector union) and NUBE (private sector union). Data was collected from a random sample of 402 members and a survey questionnaire was employed. The findings found that factors like positive and negative union beliefs, union satisfaction and union leadership significantly influenced the union commitment dimension, e.g. affective attachment. While union satisfaction, work beliefs, socialization activities, personal growth and job autonomy were significant in explaining the variance in instrumental attachment. The findings also showed that sector has significantly moderated the relationships to some extent.