The influence of pyrophosphate or polyethylene oxide (PEO) on the aggregation and gelation of dispersions of model disklike clay particles (Laponite) is studied using light scattering and rheology. Pyrophosphate adsorbs onto the positively charged rim and inhibits bond formation between the rim and the negatively charged faces of the particles. At low concentrations of pyrophosphate the aggregation of Laponite is only retarded, without significant modification of the structure of the aggregates and gels. The decrease of the aggregation rate can be explained by an increase of the energy barrier to the formation of bonds in proportion to the pyrophosphate concentration. Addition of a large amount of pyrophosphate leads to the breakup of Laponite aggregates and gels. PEO adsorbs onto the Laponite particles and inhibits aggregation by steric hindrance. The reduction of the aggregation rate depends on the molar mass and is maximal at about 1000 g/mol. Higher molar mass PEO bridges between the particles and leads to the formation of clusters or a weak gel immediately after mixture.