The behaviour of aqueous calcite dispersions in the presence of sodium oleate with and without n-heptane has been investigated. Calculations based on the classical Deryaguin, Landau, Verwey, Overbeek (DLVO) theory show that there is an energy barrier between the calcite particles for both systems. This result does not correlate with experimental findings. Inclusion of a hydrophobic interaction energy term in the total potential energy of both systems markedly improved the agreement between theoretical and experimental data. It is further shown that hydrophobic interactions play a dominant role in the aggregation of particles.