The main purpose of this investigation was to find suitable methods for on-line monitoring of the contamination level in hydraulic oils in order to avoid costly breakdowns for both industrial plants and mobile machines. Three on-line sensors, available on the market, based on different methods have been investigated. These are light obscuration, filter mesh obscuration and electrical resistance increase. The sensors have been tested in a test rig, which was specially designed to avoid particle sedimentation. Two kinds of contaminations have been used: sand powder and iron powder. Additionally, in some tests, particles were generated by severe overloading of the pump. Several mass concentrations have been used to test the sensitivity and linearity of each method. The linearity was acceptable in most cases. The sensitivity is different for each method, though also acceptable. It has been found that sand powder is much more abrasive than iron power and easier to indicate with the electrical resistance increase method. Some field tests have been performed which showed that a large mistake can be made while taking a bottle sample because of the particle sedimentation. This can be eliminated by on-line monitoring.