At JET the properties of deuterium molecules and their contribution to the total deuteron flux in the outer divertor for the Horizontal configuration in L- and H-mode conditions have been determined. For this purpose the outer divertor was observed with several high resolution spectrometers and lines of sight directed vertically with spatial resolution from the outer vertical target plate to the centre of the MKIIB GB/SRP divertor. The rovibrational population of the Fulcher bands of the deuterium molecules could both be measured for the first five diagonal vibrational transitions. A collisional-radiative model for deuterium molecules was used to convert the measured photon fluxes into local molecular D2-fluxes. With more than 70% of the total proton flux originating from molecular sources molecules make up a significant contribution to the fuelling in the divertor. This was found to be in agreement with the 2d-fluid/neutral particle code EDGE2D/NIMBUS.