Plaster or gypsum (CaSO4 center dot 2H(2)O) is obtained from a suspension of beta calcium hemihydrate (CaSO4 center dot 1/2H(2)O). The effect of two additives, known to retard setting, namely citric and malic acids, on the electrical conductivity and on the acoustic emission (AE) has been studied. This work presents the correlation obtained between these two techniques and their ability to differentiate between the crystallisation of a plaster with or without an organic additive. In particular, the surface adsorption of these carboxylic acids on solid surfaces has been evidenced by a specific acoustic emission activity.