The magnetic anomaly profile across a two-dimensional block faulted at right angles to the ground surface looks very similar to the anomaly profile across a dike and can be decomposed into a symmetric part and an antisymmetric part. Following Koulomzine et al (1970), these parts are separately analyzed and expressions for diagnostic parameters derived. The values of these parameters, obtained from the components of a field profile, help in distinguishing the source (dike or faulted block) and in determining the geometry of the block analytically or with the aid of six master curves. Some features of the application and sensitivity of the method are discussed. Field examples are given from the western margin of the Perth basin in Western Australia and from Lachlan foldbelt in New South Wales.