Global positioning systems like GPS or GALLILEO will soon provide a very good timing accuracy, making possible the synchronization of nodes in a mobile ad hoc network (MANET). With this assumption, TDMA based MAC protocols can provide a very good utilization of the shared radio resources. This paper presents an analytical model for the performance evaluation of slotted MAC protocols with reservation for MANETs. A fully connected network is assumed and nodes generate a ON/OFF exponential traffic. The analysis is based on the study of a discrete time Markov chain. The methodology is applied to a recently proposed protocol, called CROMA, but can also be applied, with suitable modifications to any slotted protocol with reservation.