...
Transliteration may be reversible in the case of alphabetic writing systems etc., as long as no letter in either script is represented by more than one letter in the other script. Thai, for example, is probably not reversible as it frequently represents the same sound with different letters depending on the tone of the syllable. See, for example, the high and low letters kho here: http://en.wikipedia.org/wiki/Thai_alphabet#Alphabetic