Since an alcohol free formulation of the EO have come up to the market, it seems convenient to compare its effects to the traditional one. Although some studies comparing the effects of T-EO and Af-EO have been found, none of them assessed and compared their substantivity and antiplaque effect in an in vivo model of structured oral biofilm. For this reason, the aim of the present study was to compare the in vivo antibacterial effect (immediate effect, substantivity and antiplaque effect) of the EO with and without alcohol in structured oral biofilm.