In this paper we present our recent work to develop phonemic and syllabic inventories for Castilian Spanish based on the C-ORAL-ROM corpus, a spontaneous spoken resource with varying degrees of naturalness and in different communicative contexts. These inventories have been developed by means of a phonemic and syllabic automatic transcriptor whose output has been assessed by manually reviewing most of the transcriptions. The inventories include absolute frequencies of occurrence of the different phones and syllables. These frequencies have been contrasted against an inventory extracted from a comparable textual corpus, finding evidence that the available inventories, based mainly on text, do not provide an accurate description of spontaneously spoken Castilian Spanish.