Parameters influencing the stability of active principles of effervescent tablets containing acetylsalicylic acid were investigated. On the basis of factorial experiment project the effects of contents of citric acid: sodium hydrogen carbonate, polyvinylpyrrolidone and those of storing temperature of tablets and humidity of atmosphere on decomposition of active principle were studied. High performance liquid chromatographic method has been developed for simultaneous determination of acetylsalicylic acid and salicylic acid. It has been established that first of all storing circumstances effect the stability of preparation and composition of tablets also significantly influences the decomposition of acetylsalicylic acid.