An economic utility theory model specifying automobile ownership decision-making in terms of intra-urban locational factors is presented. The long-range objective of the research is to provide an improvement in urban transportation planners' understanding of the relationships between automobile ownership by families residing within urban environments and the supply of transportation system facilities and services. Determinant variables in the model are: the spatial locations of trip destinations accessible to urban travellers with and without use of an automobile and at various times of day, the respective travel times to these destinations by auto and by alternate public transit modes, and the yearly capital and operating cost of automobile ownership in excess of cost by the alternate mode(s). Explicitly evaluated factors in the theory of decision-making proposed include the perceived benefits to tripmakers of travelling to destinations at various times, and the values placed on leisure time and consumption of all other goods. Emphasis is placed on decisions to purchase a second car, predictions of which are particularly relevant to urban transportation planning. Empirical tests for application with traditional transportation planning data are developed for various model hypotheses. These hypotheses are interpreted in terms of future research needs and directions.