AbstractIn this paper we compare addition reactions to ethoxyethyne with those to ethylthioethyne, in order to show up similarities and differences with respect to the orientations. The similarities are due to the similarity in character of OR and SR groups. Both groups allow the following polarisation:Electrophilic reagents are therefore added to the β‐carbon atom. The differences, on the other hand, concern additions of nucleophilic reagents BH. Ethoxyethyne, polarised as above, yields adducts of the type:Ethylthioethyne, however, is converted into products of the type:This important difference is explained by the ability of sulphur to accomodate a decet of electrons in its valence shell. It can therefore also act as an electron acceptor, allowing reversed polarisation of ethylthioethyne thus:This is especially favourable for reactions with nucleophilic reagents and results in the observed mode of addition.Differences between the effects of alkoxy and alkylthio groups encountered in the literature can be understood from this dual nature of alkylthio grou