In this work the exchange and competitive adsorption of polyelectrolytes on crystal surfaces has been investigated. Previous work had established that, in contrast to uncharged polymers, when polyelectrolytes were adsorbed onto crystals, low-molecular-weight molecules were preferentially adsorbed. Competitive adsorption experiments onto CaCO3 using sodium polyacrylate (NaPA) and sodium poly(styrene sulphonate)(PSS) have shown that in water PSS is excluded from the CaCO3 surface by the NaPA, although in the presence of excess electrolyte the polymers adsorb simultaneously. However, where PSS was pre-adsorbed, addition of NaPA in water does not cause the PSS to desorb, although in excess electrolyte added NaPA does cause some PSS desorption. The results are explained by the weaker adsorption of PSS than NaPA onto CaCO3 and the establishment of repulsive electrostatic potentials following adsorption of polyelectrolytes. This potential can prevent exchange of polyelectrolytes, thus allowing the initial preferential adsorption of low-molecular-weight polyelectrolytes to be maintained for long times.