The present invention relates to effervescent compositions which are resistant to water vapour in the atmosphere and to methods of preparing such compositions. In particular the invention relates to an effervescent composition comprising a co crystal. The co crystal comprises an acidic component and a basic component is separate. The co crystal comprising the acidic component is resistant to water uptake avoiding initiating the effervescence prematurely. Upon dissolution of the co crystal and the basic component effervescence occurs.
