Occupational exposure to 50 Hz electric and magnetic fields (EMF) was measured among 184 workers in thermoelectric power production plants using an individual portable dosimeter. A job exposure matrix (JEM) is elaborated from these data to be used in an epidemiological study on the potential carcinogenic effects of EMF. To reduce the range of exposure misclassification in the study, groups of workers with high exposure homogeneity must be identified. Classifying the workers by type of plant yielded homogeneous exposure groups, especially for workers in non-nuclear power plants. Workers in nuclear plants had higher mean exposure to magnetic fields, but the homogeneity of exposure was smaller. The exposure also differed between occupations, but the occupational title did not produce a uniform increase of exposure homogeneity within subgroups. It is concluded that the place of work is the most important determinant of exposure to magnetic fields for workers in thermoelectric power plants to be included in the JEM.