Some embodiments include methods of removing silicon dioxide in which the silicon dioxide is exposed to a mixture that includes activated hydrogen and at least one primary, secondary, tertiary or quaternary ammonium halide. The mixture may also include one or more of thallium, BX3 and PQ3, where X and Q are halides. Some embodiments include methods of selectively etching undoped silicon dioxide relative to doped silicon dioxide, in which thallium is incorporated into the doped silicon dioxide prior to the etching. Some embodiments include compositions of matter containing silicon dioxide doped with thallium to a concentration of from about 1 weight % to about 10 weight %.