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A general survey of the reaction of electrophiles with mono-olefins and conjugated dienes has led to the realization that in certain cases, subtitution may take place concurrently with addition. This is thought to be important in the understanding of the mechanism of this class of reaction especially as to the nature of the intermediate. Therefore, the additions of hydrogen chloride and of chlorine to a simple unsymmetrical conjugated diene, penta-1,3-diene ,were studied under suitable conditions. The products of these reactions were investigated, identified and characterised as far as possible, particularly by using vapour phase chromatography. The only product detected from hydrochlorination of penta-1,3-diene was 4-chloro-pent-2-ene formed by 1,2-and 1,4-addition. The chlorination of penta-1,3-diene, with the hydrocarbon in excess, gave three monochlorides and two dichlorides; when technical penta-1,3-diene was used, 1,2-dichlorocyclopentane was also found to be present because of the presence of cyclopentene as an impurity. The monochlorides obtained were shown to be 4-chloro-pent-2-ene, 1-chloro-penta-1,3-diene, and 5-chloro-penta-1,3-diene. The dichlorides were probably 1,2-dichloro-pent-3-ene and 1,4-dichloro-pent-2-ene. The product distribution was found to be depend on the temperature of the reaction, the solvent, and the relative concentration of reactants. These results were interpretable in terms of the existingtheories of the probable reaction mechanism. A stepwise reaction with an allylic carbonium ion intermediate were thought to be involved in both substitution and addition. Evidence is presented that free-radical mechanisms are not involved. In the addition of hydrogen chloride to penta-1,3-diene, the initial addition of the proton was at the end of the dienic chain, thus forming a carbonium ion; this was followed by addition of chloride anion to the 2 and 4 carbon atoms to give the product. In chlorination, the carbonium ion resulting from the electrophilic attack of the chlorine cation followed two pathways: (a) loss of a proton to give substitution products: two of the monochlorides (1-chloro-penta-1,3-diene and 5-chloro-penta-1,3-diene); (b) addition of chloride anion to give the two dichlorides. The presence of 4-chloro-pent-2-ene in the chlorination product was probably due to the reaction of penta-1,3-diene with hydrogen chloride which was formed during the reaction. The reference compounds, namely, 4-chloro-pent-2-ene, 1-chloro-penta-1,3-diene, 5-chloro-penta-1,3-diene and 4-chloro-pent-1-ene were all prepared by independent methods in a sufficiently pure state for comparison.