The adsorption of an anionic polyelectrolyte, sodium poly(styrenesulfonate), at the interface between a bulk solution, or subphase, and a cationic insoluble monolayer has been studied by means of neutron reflection, pressure/area isotherm, and surface potential measurements. At a low subphase ionic strength both Coulombic interactions between a polyelectrolyte segment and the monolayer head group and entropically favored mixing of segments with monolayer material occur. However, at a high subphase ionic strength, Coulombic interactions are less important than segment/monolayer mixing.