A system for the purpose of condition monitoring of gearboxes includes processing equipment that is configured to receive data that contains information of a spectrum of mechanical vibrations measured from a gearbox being monitored. The processing equipment is configured to search, from the spectrum of mechanical vibrations, at least two spikes on such frequencies which correspond to a phenomenon such as for example a bearing fault. The phenomenon is defined with at least two phenomenon-specific frequencies and with reference values of amplitudes relating to these phenomenon-specific frequencies. The processing equipment is configured to form an indicator of probability of occurrence for the phenomenon on the basis of amplitudes of the searched spikes and the reference values of the amplitudes. The fact that the indicator is formed on the basis of amplitudes of two or more spikes which relate to this phenomenon improves the reliability of the indicator.