IN many substances, changes in chemical composition, pressure or temperature can induce metal-to-insulator transitions(1). Although dramatic changes in optical and electrical properties accompany such transitions, their interpretation is often complicated by attendant changes in crystallographic structure(2). Yttrium, lanthanum and the trivalent rare-earth elements form hydrides that also exhibit metal-insulator transitions(3-5), but the extreme reactivity and fragility of these materials hinder experimental studies(5,6). To overcome these difficulties, we have coated thin films of yttrium and lanthanum with a layer of palladium through which hydrogen can diffuse. Real-time transitions from metallic (YH2 or LaH2) to semiconducting (YH3 or LaH3) behaviour occur in these films during continuous absorption of hydrogen, accompanied by pronounced changes in their optical properties, Although the timescale on which this transition occurs is at present rather slow (a few seconds), there appears to be considerable scope for improvement through the choice of rare-earth element and by adopting electrochemical means for driving the transition, In view of the spectacular changes in optical properties-yttrium hydride, for example, changes from a shiny mirror to a yellow, transparent window-metal hydrides might find important technological applications.