1. This article reconstructs six pairs of PSl equivalents for ‘alder' preserved in Slavic dialects in an appellative and/or onomastic function, whose elements can be distinguished from one another by grammatical gender (feminine or masculine) and declension type: *olьxa - *olьxъ, *olьša - *olьšь, *olьs'a - *olьs'ъ, *elьxa - *elьxъ, *elьša - *elьšъ, and *elьs'a - *elьs'ъ. The masculine forms are known only in East and South Slavic dialects. 2. The elements of the pair with the oldest phonetic form, which show the word-formational relationship of an ā-stem for the feminine form (*alisā) versus an o-stem for the masculine form (*aliso-s), can be dated at least to early PSl; other phonetic pairs developed later because they contain younger phonetic forms. 3. The base pair (*alisā ‘alder' - *aliso- ‘alder?') shows the same word-formational and morphological relationship as certain other expressions for trees derived from stems denoting colors (e. g., PSl *berzā - *berzo- ‘birch', *berstā - *bersto- ‘elm'). 4. The masculine and feminine elements of all pairs are phonetically equal, refl ecting the same principles of Slavic phonetic development (except for the oldest phonetic change at word onset manifested in Slavic *o- : *(j)e-, which cannot be unambiguously explained): cf. the pan-Slavic development of PIE *s > PSl. *x (*oalisā > *oalixā or *ealisā > *elixā), the pan-Slavic development of PIE *i > PSl. *ь (*oalixā > *olьxa, *ealisā > *elьxa), and the development of PSl *x > s' after progressive palatalization (*oalixā > *olьs'a, *ealisā > *elьs'a) in East and South Slavic dialects. In contrast, the -š- in the West Slavic forms *olьša, *jelьša is explained as a structural element rather than the West Slavic refl ex of the progressive palatalization of PSl. *x > *š' (*oalixā > *olьš'a, *ealisā > *elьš'a), as would be expected. The reason for this should be sought in the South Slavic phonetic forms of the type *olьša and *elьša, in which the appearance of -š- could imply morphological generalizations from various derivatives. 5. The PSl presence of phonetic variants with a sibilant *s' < PSl *x (i. e., *elьs'a and *elьs'ь) in South Slavic is indicated by rare lexical vestiges from the periphery of the vast linguistic area extending from eastern Prekmurje and Prlekija in Slovenia to Kajkavian territory (i. e., the area around Varaždin, Kotoriba, Gola to the east of Koprivnica, the area around Zagreb, and the southern edges of Slavonia in Croatia) as well as the Čakavian area (i. e., Brinje in the Lika region, and Jelsa on the island of Hvar in Croatia). 6. The phonetically innovative South Slavic forms *elьs'a, *elьs'ь alongside archaic *elьxa, *elьxъ can be compared to the phonetically innovative East Slavic forms *olьs'a, *olьs'ь alongside archaic *olьxa, *olьxъ, and the *s' can be defi ned as an East or/and South Slavic refl ex of *x after progressive palatalization, which extends well into the PSl period and, alongside PSl *vьxъ (East and South Slavic *vьs'ь and West Slavic *vьš'ь), is another lexeme with progressive palatalization of *x in the stem.