Objective: The study examined the factorial validity of the Maslach Burnout Inventory (MBI) for social work research and practice. Method: Factor structure and longitudinal invariance of the MBI were tested using structural equation modeling techniques with a random sample of 475 state-registered social workers in California. Results: The original three-factor model was superior over other competing factor models. Investigation of the second-order factor model supported the presence of the common burnout factor and indicated depersonalization and emotional exhaustion were core components of burnout. Longitudinal factorial invariance was not achieved for personal accomplishment. Conclusion: The findings suggest the applicability of the MBI in a longitudinal research with careful inference regarding personal accomplishment and highlight the importance of depersonalization in social worker burnout.