This paper presents an acoustic phonetic study of Polish V#V sequences designed to shed light on the phonological representation of glottal marking. Independent phonological evidence from Polish suggests that initial vowels contain an "empty onset" that may be realized as glottal marking. The results of the experiment suggest that glottal marking in Polish is quite robust, and may be realized by increases in spectral balance. In the Onset Prominence environment, the "empty onset" is derived from phonetic principles, realized as specification for the Vocalic Onset layer of structure. VO parameter settings capture important ambiguities in speech perception and allow for a unified analysis of glottal marking, distributional restrictions on Polish vowels, and ambiguities underlying palatalization processes.