A model of the determination of the union status of workers is developed that incorporates the separate decisions of workers and potential union employers and recognizes the possibility of an excess supply of workers for existing union jobs. This model is estimated using data from the Quality of Employment Survey that have a unique piece of information on worker preferences which allows identification and estimation of the model. The empirical results yield some interesting insights into the process of union status determination which cannot be gained from a simple logit or probit analysis of unionization.