We explore a technique for decelerating molecules using a static magnetic eld and optical pumping. Molecules travel through a spatially varying magnetic fi eld and are repeatedly pumped into a weak-field seeking state as they move towards each strong field region, and into a strong-fi eld seeking state as they move towards weak fi eld. The method is time-independent
and so is suitable for decelerating both pulsed and continuous molecular beams. By using guiding magnets at each weak field region, the beam
can be simultaneously guided and decelerated. By tapering the magnetic field strength in the strong field regions, and exploiting the Doppler shift, the velocity distribution can be compressed during deceleration. We develop the principles of this deceleration technique, provide a realistic design, use numerical simulations to evaluate its performance for a beam of CaF, and compare this performance to other deceleration methods.