Boron and phosphorus passivation in silicon metal-oxide-semiconductor (MOS) capacitors under x-ray irradiation follows a power law in the form ΔN/N₀~(dose)^α, with ½≤α≤1, depending on processing parameters and the device history before irradiation. The minimum x-ray dose for observation of dopant passivation in fluorinated p-type MOS capacitors is smaller when the time interval between fabrication and subsequent irradiation increases. We reason that room-temperature ageing within this timescale is a result of either adsorption of water or gradual reactions of fluorine with hydrogen, silicon or oxygen-related species in the neighbourhood of the SiO₂/Si interface.