This paper investigates the impact of contamination of engine lubricants with ethanol fuel on the growth and stability of anti-wear tribofilms from ZDDP-containing lubricants. The MTM-SLIM technique was used to monitor the effects of blending 5 wt% of both anhydrous and hydrated ethanol on tribofilm thickness in a fully-formulated Group I oil and in a solution of ZDDP anti-wear additive dissolved in Group I base oil. Tribofilm thickness was significantly reduced by the addition of ethanol for both oils, and the reduction was more severe for hydrated than for anhydrous ethanol. When a tribofilm was allowed to form during rubbing using an ethanol-free oil, the subsequent addition of hydrated ethanol showed both the destruction of the pre-formed antiwear tribofilm and damage to the rubbed surfaces.