Different thematic realizations of the same concept (e.g. passive, unaccusative, reflexive, reciprocal) are assumed to be derived from the same basic entry via thematic arity (valence changing) operations. The cross-linguistic variation in deriving new predicates via arity operations, is accounted for by the Lex(icon)-Syn(tax) Parameter (Reinhart & Siloni 2005), according to which UG allows arity operations to apply in the lexicon or in the syntax. Reinhart and Siloni (2005) show a cluster of semantic-syntactic properties that result from this parameter. I argue that the Lex-Syn parameter has a morpho-phonological manifestation as well. Based on the Stem Modification approach (Steriade 1988, McCarthy & Prince 1990, Bat-El 1994), I examine the morpho-phonological alternations within the verbal systems of Modern Standard Arabic, Palestinian Arabic and Modern Hebrew. I show that syntactic and lexical operations differ with regard to three morpho-phonological properties: intrusivness of the morpho-phonological process, directionality and predictability.