Holocene clays that contain a significant amount of sulphide are regularly discovered during site investigation programs carried out along the East coast of Sweden. Cone Penetration Tests (CPTs) are frequently executed during such investigations, and the mechanical properties of the soil is subsequently interpreted from the CPTs. The mechanical behaviour of the sulphide clays is somewhat similar to other high-plasticity Scandinavian clays, including shear-strength anisotropy, and a high level of strain-rate and temperature dependency. These factors make the interpretation of the soil properties complicated, since the testing conditions and the testing methods have a large influence on the soil behaviour. Sample disturbance and the temperature of laboratory direct simple shear tests also influence the interpretation, since these have been used as a reference for in-situ tests. The boundary conditions of the soil during both the in-situ and laboratory tests are consequently mixed in the interpretation, resulting in significant uncertainty about the correct soil properties. The design shear strength is typically chosen conservatively, but studies show a significant variation in the resulting cone factors for sulphide clays from empirical correlations. There are however some cases where a low undrained shear strength is not conservative, and in which design guidelines create some confusion. A case study of Cone Penetration tests in sulphide clay in Eastern Sweden is examined and some factors which influence the soil behaviour are discussed, including the strain-rate, temperature and anisotropic strength.