Periodic macroporous forms Of nC-SnO2 have been synthesized by two methods, giving opals and inverse opals that can be used as structurally well-defined gas sensors, as demonstrated for CO gas, as well as for toluene and ethanol vapors. The inverse opals, in particular, seem to approximate "ideal" behavior. The Figure shows an SEM image of the amorphous SnO2 inverse opal structure after toluene extraction.