The neuter interrogative pronouns el qué and lo qué alternate with qué ‘what’ in conversational Spanish in certain contexts and types of wh-questions (e.g., – No me lo puedo creer – ¿No te puedes creer qué/el qué/lo qué?). This paper describes geographical, register and social variation in the use of el qué and lo qué and analyses the interpretive and distributional properties of these complex forms. Building on corpus data and speakers’ judgements, it is shown that the form el qué is common to all Spanish dialects and is used in all registers and at all social levels, whereas lo qué is only found in Spain, Argentina and Uruguay: in Spain and Argentina, this form belongs to lower class people with little education and speakers in rural areas; in Uruguay, it does not have a social bias and occurs in both formal and informal situations. It is also argued that el qué and lo qué are used to indicate that the target of the question is identifiable in the context of utterance, and it is thus claimed that el and lo in el qué and lo qué are overt definiteness markers.