When an acid HA adds to an alkene, a mixture of products can be formed if the alkene is not symmetrical. For instance, the reaction between C₂H₅CH=CH₂ and HCl can give C₂H₅CH₂CH₂Cl or C₂H₅CHClCH₃. In general, a mixture of products occurs in which one predominates over the other. In 1870, Vladimir Markovnikoff (1837–1904) proposed the rule that the main product would be the one in which the hydrogen atom adds to the carbon having the larger number of hydrogen atoms (the latter product above). This occurs when the mechanism is electrophilic addition, in which the first step is addition of H⁺. The electron-releasing effect of the alkyl group (C₂H₅) distorts the electron-distribution in the double bond, making the carbon atom furthest from the alkyl group negative. This is the atom attacked by H⁺ giving the carbonium ion C₂H₅C⁺HCH₃, which further reacts with the negative ion Cl⁻.

In some circumstances anti-Markovnikoff behaviour occurs, in which the opposite effect is found. This happens when the mechanism involves free radicals and is common in addition of hydrogen bromide when peroxides are present.