An electrochromic data recording device such as a disc including at least one electrochromic zone (4) in and as a part of an electrical cell (2, 4, 5, 6, 7) for selectively supplying and withdrawing guest atoms to and from the zone to produce an optical change in the zone, the cell being substantially inoperable at a first temperature but being operable at a temperature elevated with respect to the first temperature by virtue of the mobility of current carrying ions in a component of the cell which are appropriately temperature dependent. The device can be written on by laser heating the device locally to allow current to pass to produce a local optical effect and can be read by a laser and photodector in combination. Further, the device can be erased by being heated sufficiently to become conductive.