The paper deals with the following morphophonological aspects of prefixation in modern Spanish: the accentuation of prefixes or prefixoids in combination with Greco-Latin roots; the vocalic fusion at the boundary between two Greco-Latin constituents; the selection of termination in prefixoids of Latin or Greek origin; a certain fluctuation regarding the use of orthographic marks of separation between the prefix/prefixoid and the root. Our investigation shows that the majority of these phenomena are not conditioned by the derivative or compositive nature or other characteristics of the prefixal morpheme, as suggested erroneously by some Spanish linguists, but result from specific properties of the word stem.