A job exposure matrix combining features to increase the accuracy of exposure assessment was developed to evaluate cancer risks from workplace exposures to six chlorinated aliphatic hydrocarbons (CAHs). A detailed description of the matrix is provided to satisfy the need for more in-depth discussion of exposure assessment methods than is typical in today's epidemiologic literature. The matrix assigns semiquantitative estimates of the probability and intensity of exposure to each four-digit Standard Industrial Classification (SIC) and Standard Occupational Classification (SOC) code potentially associated with exposure to each CAH. The matrix also accounts for the changing patterns of use of the CAHs by decade from the 1920s to the 1980s. An algorithm combines these parameters to assign each study subject a unique lifetime probability of exposure and an estimated score of cumulative exposure for each CAH. These assignments can then become the subjects of analyses. The ability of the matrix to reduce the number of false positive exposure assessments is discussed and illustrated. A companion paper describes the detailed epidemiologic findings of this application of the matrix.