The article presents a glottochronological method for determining the coefficient of diachronic speed of decomposition of the Old English synonymic set “scip”. Lexical units of the synonymic set “scip” in the early Middle English period are analyzed on the basis of decomposition equations. Depending on the degree of preservation of the word semantic potential, the decomposition rates for the preserved words are calculated both on the lexical and semantic level. In general, the lexicostatistical method of glottochronology allows us to understand more deeply not only the external side of historical and linguistic processes associated with the emergence and functioning of lexical phenomena, but also the internal nature of the linguistic process in the diachronic aspect.