Abstract: In a multi-channel electrical communication system 1, a method of estimating the interference level in a test channel due to the intermodulation of two or more other channels, where each channel is centred upon a carrier frequency. The method comprises, for a given m-th order mode, an initial step of identifying the set(s) of carrier frequencies, the m-th order linear combination of which gives rise substantially to the carrier frequency of said test channel. This process is carried out by a computer 2. For the or each of the identified set of carrier frequencies, signal generators 3 are tuned to respective ones of the carrier frequencies. The generated signals are applied to an input of the communication system 1, and the intermodulation interference at the carrier frequency of the test channel is measured at an output of the system 1.