The dependence of the measured thickness of Langmuir-Blodgett monolayers of various amphiphiles on the force exerted by the scanning tip was investigated with scanning force microscopy, both in the contact and the tapping mode. There is clear evidence for the dependence of the apparent film height on the applied force: at first, the measured film thickness decreases rapidly with increased force; then it stays constant forming a plateau region before it drops again and reaches a minimum value of a few angstroms. Only in the contact mode by minimized force the correct film thickness could be measured. This qualitative curve can be obtained from all kinds of monolayers investigated here. The exact shape of height-force curve is determined by the properties of the molecules and the preparation conditions. A possible explanation for this behavior is given by a molecular model based on gauche defects.