と思いきや|とおもいきや

Marks an assumption the speaker held and shows that the actual outcome turned out contrary to it. It means "I thought …, but in fact …" and introduces a surprising result.

A literary, somewhat dramatic expression typical of narratives, essays, columns and commentary. It is also heard in speech, such as storytelling and broadcast commentary, and is used playfully in casual writing. The first clause states what one assumed, and the second reports the contrary fact, usually in the past tense. Unlike plain contrast connectors, it foregrounds the gap between expectation and reality, often with surprise.

• かと思ったら / かと思うとN2「かと思ったら」is close to this pattern when it means 'I thought... but actually...', and the two are often interchangeable, but it is more neutral in tone and common in speech. 「かと思ったら / かと思うと」can also mean 'as soon as... (right after...)' when attached to the ta-form of a verb, a meaning this pattern does not have.
• もののN2With 「ものの」, the first clause is an established fact and the second clause did not develop as expected, carrying a nuance of regret or resignation. With 「と思いきや」, the first clause is only a subjective expectation and the second clause is the actual situation contrary to it, with a sense of surprise. The two cannot be interchanged.
• にもかかわらずN2With 「にもかかわらず」, the first clause is an established fact, while with 「と思いきや」it is only the speaker's expectation. 「にもかかわらず」can also be followed by a request or intention, with an objective, formal tone, so the two cannot be interchanged.
• とはいえN1With 「とはいえ」, the first clause is an established fact and it means 'although... / that said...', with the second clause able to carry a judgment or reminder, and it can also stand alone at the start of a sentence. With 「と思いきや」, the first clause is only an expectation and the second clause must be an unexpected actual result, so the two cannot be interchanged.
• 逆にN2「逆に」is an adverb or conjunction, neutral in tone, placed at the beginning of the second sentence or before the predicate. Only its 'on the contrary / rather' meaning is close to this pattern, but this pattern follows an expectation to express surprise, while 「逆に」can also express a contrast between two sides, so it cannot directly replace this pattern.
• ところがN3「ところが」is mostly used at the start of a sentence to connect two sentences, and can also attach to the ta-form of a verb, with a neutral tone; the expectation may be stated in the first sentence or conveyed only by context. 「と思いきや」attaches directly to the content of the expectation and leans toward written style. When the first sentence states the expectation, the two can be rewritten as each other.

かと思ったら／かと思うと can also mark an overturned expectation, but they are more conversational and can also describe one event immediately following another, which と思いきや cannot. ものの, にもかかわらず and とはいえ concede an established fact rather than a mistaken assumption, while 逆に and ところが are standalone connectors; ところが can express surprise, but it lacks the explicit "I thought" framing.