3He atoms dissolved in superfluid 4He may form dimers (He)2 in two-dimensional (2D) geometries. We study dimer formation in films of dilute 3He–4He mixture. After designing a schematic 3He–3He interaction potential we calculate the dimer binding energy for various substrates. It is shown that 3He impurity states localized near the substrate give rise to the largest magnitudes of the binding energies. PACS numbers : 67.70.+n Helium films. 67.60.Fp He II – 3He mixtures. IPNO/TH 94-56 to appear in J. Low Temp. Phys. 1 Unité de Recherche des Universités Paris XI et Paris VI associée au CNRS.