Zeeman splitting of spectral lines from hydrogen and impurities in the JET tokamak plasma has been observed in the visible region of the spectrum. An Optical 'Multi channel Analyser (OMA) coupled to a 1m Czerny-Turner instrument is used to record several spectral lines simultaneously many times during a discharge. Tangential viewing optics enhance the influence of Zeeman splitting, especially when the impurities are located where the magnetic field is aligned along the viewing chord. To date, splitting of Ha (6563Å), CII (6578Å and 6583Å), Cri (4289Å, 4274Å and 4254Å) and Cl II (4794Å, 4810Å and 4819Å) has been detected. The location of the impurities is determined from the magnitude of the splitting and the known toroidal field distribution. The effects of the magnetic field on the line profiles are best observed during the plasma initiation and termination phases where, for example, a large plasma incursion can result in the injection of impurities from the inner wall. The technique is of particular importance in determining the source of impurities (e.g. from the inside wall or the limiter at the outer periphery). The observations also highlight the importance of taking Zeeman splitting into account when making Doppler shift or broadening measurements in the visible spectrum.