This report establishes that the test (the UN N.4 test) used for classifying dangerous goods prior to transport does not produce reliable results. The test can yield both false negatives (materials classified as non-self-heating, although they are) and false positives (materials classified as self-heating, although they are not). While false positives do lead to increased transport costs, they do not constitute a safety issue. False negatives, however, represent a direct threat to fire safety, as there is a risk of self-heating followed by spontaneous ignition. The report focuses on the maritime transport of solid materials and begins by outlining classification of dangerous goods and solid bulk cargoes from a maritime perspective, with particular emphasis on the International Maritime Dangerous Goods (IMDG) Code and the International Maritime Solid Bulk Cargoes (IMSBC) Code. A cornerstone in determining the self-heating propensity of a material under the IMDG Code is the UN N.4 test. This test is also referenced in the IMSBC Code, although with different temperature requirements compared to its original formulation. Although the focus on maritime transport in this report, the UN N.4 test is also applied in the classification of dangerous goods in land transport (ADR/RID), and the findings of this report may therefore be relevant to a broader audience. The risk of the UN N.4 test producing misleading results stems from the fact that different substances exhibit different behaviours/reactions during self-heating and that the UN N.4 test originates from one type of material’s behaviour. The rate of the reactions depends, among other factors, on temperature. Since different substances exhibit different temperature dependencies in their reaction rates, no single test temperature that can adequately cover all cases. Experiments must therefore be conducted at multiple temperatures. The report presents a number of methods for determining the self-heating rate (i.e., the propensity for self-heating) of a substance. Once critical constants (e.g., apparent activation energy, pre-exponential factor, and material dependent constants) have been established, these can be used to estimate critical parameters such as temperature and the size/volume of the material to be transported. Finally, the report proposed an improvement to the UN N.4 test. The proposed method is based on the crossing-point method and determination of heat capacity, heat conductivity and density. From this, material specific properties can be determined and used to calculate the critical volume at 50 °C.