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 (C6−C10) 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.