The present paper investigates the grammatical properties of interrogative verbs in Kavalan and Amis, two Formosan languages. The interrogative words that denote 'do what', 'what happen', 'do how', '(put/take) where', and '(do) how many/much' in the two languages share the same morphosyntactic distribution as verbs. Their transitivity is correlated with the voice markers affixed to them. The affixation of the agent voice marker to an interrogative root forms an intransitive verb, whereas an interrogative root with the patient voice marker is interpreted as a transitive verb. Some interrogative verbs can occur in a verb sequencing construction where they precede a lexical verb. It is argued that the two verbs in this construction do not form a coordinate structure, but exhibit properties of subordination with the interrogative verb as the main verb. Finally, there are semantic restrictions on the use of interrogative verbs. Kavalan tanian and Amis icuwa '(put) where' show verbal properties only when they question the location of a theme argument in a ditransitive event, but not the location where an event takes place. When Kavalan tani and Amis pina/hakuwa '(do) how many/much' are used as verbs, the question always implies that the quantity of the questioned argument will or might change eventually.