The role of long range dispersion forces plays a significant role in the adsorption of weakly chemisorbed molecules, as demonstrated by our first-principles calculations with the van der Waals density functional (vdW-DF) applied to the model system salicylic acid (SA) on the NaCl(001) surface. While the vdW interaction accounts for more than half of the adsorption energy, the equilibrium geometry is still mostly determined by the chemical interaction between O and Na atoms. This seems to be due to a compensation effect of the vdW interaction in different parts of the planar SA molecule. The inclusion of the vdW interaction also results in a closer alignment of the highest occupied orbital of SA and the valence band maximum of NaCl(001) as a consequence of stronger hybridizations.