The attraction forces responsible for the occurrence of limited swelling of montmorillonite having divalent counterions were evaluated from (a) the electrostatic attraction between the negative charges of one surface and the effective charges of the counterions of the other interlayer surface and (b) van der Waals attraction between the two interlayer surfaces. Both methods of calculation almost account for the force required to maintain limited swelling of the clay. It is found that a Hamaker constant of 2.25 x 10(-13) erg in place of 2.72 x 10(-13) erg and a counterion charge screening factor of 0.82[1 -exp(-kappa x)] in place of [1 - exp(-kappa x)] reproduce the true value of the attraction force.