A critical issue in mobile data management is to respond to real-time data access requirements of the supported application. However, it is difficult to handle real-time constraints in a mobile computing environment due to the physical constraints imposed by the mobile computer hardware and the wireless network technology. In this paper, we present a mobile database system model that takes into account the timing requirements of applications supported by mobile computing systems. We provide a transaction execution model with two alternative execution strategies for mobile transactions and evaluate the performance of the system considering various mobile system characteristics, such as the number of mobile hosts in the system, the handoff process, disconnection, coordinator site relocation and wireless link failure. Performance results are provided in terms of the fraction of real-time requirements that are satisfied.