Satellite-based soil moisture measurements indicate a diurnal variation in backscatter in west Africa. This diurnal variation is assumed to be caused by variation of moisture content in vegetation (Friesen, 2008). The obtained satellite soil moisture data can be improved, if ground measurements support this hypothesis. Especially at locations with a dense land cover, where the satellite can’t penetrate into the soil improvements can be made. Currently there are no easy applicable methods to measure vegetation water content continuously, which can assist in confirming the relation between vegetation water and the satellite measurements. In this research a new non-destructive, cheap, easy applicable and continuous method to measure water content in trees is developed. This new method makes use of the variation in natural frequency caused by the diurnal variation of water content in trees. The natural frequency of a tree depends on its stiffness and its mass. Diurnal variations in the mass of a tree are mainly caused by changes in water content. To determine the natural frequency two methods are tested. The first method makes use of the wind as a driving force and the measurements are analysed in the frequency domain. The second method uses a single pulse as driving force and the measurements are analysed in the time domain. To confirm the validity of the proposed analyses both methods are first tested on simplified system (a stick fixed in a vice). Different weights are attached to the top of the stick to relate weight and frequency changes. The vibration of the simplified system and of trees are measured with an accelerometer. To determine the natural frequency both systems are modelled as a damped first order spring-mass system. Results indicate that, for the simplified system, changes in natural frequency are detectable and can be related to mass changes of the system. When the methods are applied on a tree the accuracy of the measurements is not high enough to identify changes in weight, caused by variation in water content. It is possible to measure the natural frequency of a tree but the changes in natural frequency due to changes in water content are very small (less than 0.1 Hz for small trees). For the simplified system the weight changes compared to the own weight of the system are much larger. Presumably, this is the main reasons for the difference in performance between the simplified system and measurements on a tree. Additionally, the simplified system is better represented by a first order spring-mass system. Previous studies have indicated that the anisotropic mechanical properties of a tree, prevents it from behaving like a first order spring-mass system.