Many spacecraft attitude control systems use actuators that operate in an on-off mode; typical examples are the jet thrusters, for which the control amplitude cannot be modulated. Moreover, if we want to reduce the complexity of the devices that drive the actuators, as for example in the case of magnetic coils, the on-off input is the simplest signal to be generated. The control strategy presented in this paper consists of pulse-modulation control according to an adaptive model reference control law that minimizes a cost function. The cost function is a suitable Liapunov function, which takes into account the state and control variables of the controlled and reference systems. Such a control strategy was numerically tested on a momentum-bias spacecraft in the presence of environmental disturbances.