Markovnikov is best known for Markovnikovs rule, elucidated in 1869 to describe addition reactions of H-X (where X' represents a  halogen) to alkenes. According to this rule, the nucleophilic X- binds to the carbon (C) atom with fewer hydrogen atoms, while the proton binds to the carbon atom with more hydrogen atoms bonded to it. Thus, hydrogen chloride (HCl) reacts with propene, CH-CH=CH to produce 2-chloropropane CHCHClCH rather than the isomeric 1-chloropropane CHCHCHCl. The rule is useful in predicting the molecular structures of products of addition reactions. Why hydrogen bromide exhibited both Markovnikov as well as reversed-order, or anti-Markovnikov, addition, however, was not understood until Morris S. Kharasch offered an explanation in 1933. It is also called The Peroxide effect sometimes.
Hughes has discussed the reasons for Markovnikovs lack of recognition during his lifetime.  Although he published mostly in Russian which was not understood by most Western European chemists, the 1870 article in which he first stated his rule was written in German. However the rule was included in a 4-page addendum to a 26-page article on isomeric butyric acids, and based on very slight experimental evidence even by the standards of the time. Hughes concludes that the rule was an inspired guess, unjustified by the evidence of the time, but which turned out later to be correct (in most cases). A more recent assessment, based on a reading of Markovnikovs Magistr Khimii and Doktor Khimii dissertations, contradicts this view, and points out that Markovnikov's Rule arises logically from his dissertations.
Markovnikov also contributed to organic chemistry by finding carbon rings with more than six carbon atoms, a ring with four carbon atoms in 1879, and a ring with seven in 1889.
Markovnikov also showed that butyric and isobutyric acids have the same chemical formula (CHO) but different structures; i.e., they are isomers.