Poly(vinylbenzylammonium chloride) (PVBAC) polyelectrolytes possessing defined chain lengths, narrow chain length distributions, uniform charge d., but substituents of different hydrophilicity and size at the quaternary ammonium group were used as model compds. to study the influence of the chain length and the substituent type on the interaction with oppositely charged monolayers. Langmuir monolayers were formed from dimyristoylphosphatidic acid (DMPA). Pressure-area and pressure-time isotherms revealed an increase of both the area per amphiphile mol. and the surface pressure at const. area as a function of time if the polyelectrolyte adsorbs on the amphiphile. The compressibility of the DMPA/PVBAC complex layers is much higher than the compressibility of the DMPA monolayer. Also, the results clearly indicate the influence of the chain length and the substituent type. Larger and more hydrophobic substituents at the quaternary ammonium group significantly increase the area per amphiphile mol., from 0.42 nm2 for the amphiphile up to 0.97 nm2 for the complex. This parameter also increases with decreasing chain length. Incorporation of the hydrophobic substituents into the monolayer as well as end group effects probably are the reason for the monolayer extension. [on SciFinder (R)]