The effect of varying the subphase composition on the polymerization kinetics of linoleic acid monolayers was investigated. The pH of the TbCl3 subphase was found to have a dramatic influence on the compressibility of the monolayer. The appearance of a condensed-like phase at pH’s beyond a critical value was interpreted in terms of the formation of a monohydroxy complex of Tb metal ions and its interaction with the monolayer. By tuning the condensation of the monolayer a pronounced increase of the polymerization rate was observed. A model for the reaction kinetics is presented. Furthermore, the reacted monolayer could be transferred to a solid substrate for FTIR-ATR spectroscopic analysis which confirmed the W-irradiation-induced chemical changes in the monolayer structure.