This article deals with a topic to which not much attention is usually paid in the studies on phonetic variation in Spanish, since these investigations are much more frequently dedicated to syllable-final s, a variable which serves as one of the most important criteria for the classification of the varieties of Spanish. Syllableinitial s, however, in the author's opinion, deserves a more detailed study, which is why both its geographic boundaries and the conditioning factors for its appearance are studied in this paper. Moreover, different theories on the origins of this phenomenon were considered, and, based on the available data, an effort was made to establish the mechanism governing this change. [ABSTRACT FROM AUTHOR]