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An ambitransitive verb is a verb that can be used both as intransitive or as transitive without requiring a morphological change. That is, the same verb form may or may not require a direct object. English has a large number of ambitransitive verbs; examples include read, break, and understand (e.g. "I read the book," saying what was read, or just "I read all afternoon").

Ambitransitive verbs are common in some languages, and much less so in other languages, where valency tends to be fixed and there are explicit valency-changing operations (such as passive voice, antipassive voice, applicatives, causatives, etc.).

Generally speaking, there are two types of ambitransitive verbs, distinguished by the alignment of the semantic roles of their arguments with their syntactic roles.

When the subject of the intransitive form of the verb is an agent (like the subject of the transitive form), so that the verb aligns the syntactic roles S and A, then the verb is a common ambitransitive with an optional object, and the intransitive version is an unergative verb, like English eat, follow, read, win.

When the subject of the intransitive form is a patient (like the direct object of the transitive form), so that the verb aligns the syntactic roles S and O, then the verb is known as an alternating ambitransitive, and the intransitive version is an unaccusative verb, like English break, melt and sink. This means that the subject of the intransitive form corresponds with the direct object of the transitive version, so the roles are exchanged. Often depending on the linguist doing the research, the intransitive version of such a verb can be said to be in the middle voice, or to be an anticausative verb.

The term "anticausative" derives from the fact that the intransitive form of such a verb implies a deletion of the agent of causation (the reverse of a causative construction), as if the event happened by itself: when a window breaks, we know it in fact it was broken (by some person, or by some physical alteration), but the anticausative form syntactically erases the cause and also makes it impossible or difficult to refer to it (as opposed to passive voice, where the agent can usually be introduced back as an adjunct, e. g. a prepositional by-phrase in English).

Alternating ambitransitives are not uncommon in English. In the Romance languages, such verbs are rarely found, since the same semantic concept is covered by pseudo-reflexive verbs. These verbs behave like ambitransitives, but the intransitive form requires a clitic pronoun that usually serves also for reflexive constructions. See for example, in Spanish (which uses the pronoun se in the third person):

• La ventana se rompió. "The window broke."
• Este barco se está hundiendo. "This boat is sinking."
• Se derritió todo el helado. "All the ice cream has melted."

In the example, the verbs romper, hundir and derretir are all transitive; they become intransitive by using the pseudo-reflexive clitic, and the direct object becomes the intransitive subject.

Ambiguity may arise between these and true reflexive forms, especially when the intransitive subject is an animate (and therefore a possible agent). Me estoy hundiendo usually means "I'm sinking" (patientive first person), but it may also mean "I'm sinking myself", "I'm getting myself sunk" (agentive).

• Changing valency: Case studies in transitivity (edited by R. M. W. Dixon & A. Y. Aikhenvald, Research Centre for Linguistic Typology, La Trobe University, Melbourne)