According to traditional descriptive grammars, Spanish medio ('half') is a variable category. It can be a noun (medio de vida), a numeral adjective (quantifier) (media docena), or an unstressed adverb (estaba medio cansado). However, medio takes an invariable form as an adverb in standard Spanish (estaba medio cansada). Making the adverb medio agree with the adjective it modifies (estaba *media cansada) is considered nonstandard. Numerous cases are attested where the adverb medio agrees, as though it were an adjective, with the noun or the adjective which it modifies. This process, known as 'recategorization' implies that an element is interpreted as belonging to one category (i.e. adjective) in some varieties and as part of another (here, adverb) in other varieties. The 'adjectivization' of medio is widely considered to be characteristic of Latin American Spanish varieties, but it is believed to be rare in Iberian Spanish. In this study I use unpublished dialect data from the ALPI (Atlas Lingüístico de la Península Ibérica) to examine the precise geographical area of the phenomenon in Peninsular Spanish. These data clearly show that the distribution of this phenomenon is much wider than previously claimed, since it is attested not only in the Galician area but also in Leonese and in Andalusian, the latter being the probable source for the transfer of the phenomenon to Latin American Spanish. We also use historical and contemporary data from both Peninsular and Latin American texts (Corpus del español) to analyse some specific cases of adverbial medio with and without agreement (medio/media tonta), and to study the semantic interpretations associated with each variant. [ABSTRACT FROM AUTHOR]