By surface tension and ellipsometric measurements, we studied how 2D crystallization of fatty alcohol monolayers is influenced by the presence in the subphase of a short chain carboxylic acid, whose surface activity is dependent upon pH. The surface crystallization of the alcohol monolayer was appreciably perturbed for pH values smaller than the pK(a) of the acid, that is when the protonated form of the acid was predominant in the bulk solution. On the contrary, for pH values above the pK(a) of the acid, the monolayer was not influenced by the presence of the acid in the subphase. The effect was greatly dependent on the acid concentration in the subphase, at a fixed pH. We also studied the effect of the acid chain length (C-6-C-10) on a decanol monolayer, the effect of the alcohol chain length with octanoic acid in the subphase, and the effect of the lateral surface pressure of the monolayer.