All mains voltage halogen bulbs (eg GU10 and G9 types) can be dimmed but may require a dimmer of higher wattage than the total wattage of the light fitting they are dimming. All low voltage Halogen bulbs (eg G4, G6.35 and G5.3 types) may require special dimmers dependant on their transformer type and we do advise not to use dimmers with them. However, any qualified electrician should be able to advise, supply and fit these for you.